Methods and systems for high-resolution long-range flash lidar

ABSTRACT

A Light Detection And Ranging (LIDAR) apparatus includes a pulsed light source to emit optical signals, a detector array comprising single-photon detectors to output respective detection signals indicating times of arrival of a plurality of photons incident thereon, and processing circuitry to receive the respective detection signals. The processing circuitry includes one or more of a recharge circuit configured to activate and deactivate subsets of the single photon detectors for respective strobe windows between pulses of the optical signals and at differing delays, a correlator circuit configured to output respective correlation signals representing detection of one or more of the photons having times of arrival within a predetermined correlation time relative to one another, and a time processing circuit comprising a counter circuit configured to increment a count value and a time integrator circuit configured to generate an integrated time value based on the respective correlation signals or detection signals.

CLAIM OF PRIORITY

This application claims priority from U.S. Provisional Application Nos.62/630,079 filed Feb. 13, 2018, 62/637,128 filed Mar. 1, 2018,62/655,000 filed Apr. 9, 2018, and 62/684,822 filed Jun. 14, 2018,respectively entitled “Methods and Systems for High-resolutionLong-range Flash Lidar”, the disclosures of which are incorporated byreference herein.

FIELD

The subject matter herein relates generally to 3D imaging, and morespecifically to LIDAR (Light Detection And Ranging; also referred toherein as “LIDAR”) systems for 3D imaging.

BACKGROUND

3D imaging systems can be categorized as radar-based systems, which mayrely on microwave radiation (e.g., a wavelength range of 1 mm-100 cm),and optical systems, which may rely on electromagnetic radiation in theoptical band (e.g., a wavelength range of 100 nanometer(nm)-1 millimeter(mm)). Optical 3D imaging systems can be categorized to stereo-basedsystems (which may rely on the parallax effect), interferometric imagingsystems (which may rely on the Doppler effect), and Time-of-Flight (TOF)systems.

TOF 3D imaging systems can be categorized as indirect TOF or direct TOFsystems. An example of an indirect TOF system is the Photonic MixerDevice (PMD), which can measure the phase delay between a transmittedand received amplitude-modulated optical signal. The distance d to thetargets can be calculated as (d and R used interchangeably herein):

$d = \frac{C*\varnothing}{4*\pi*f}$

Phase can be detected using a quasi-CCD (charge coupled device) in-pixelconfiguration referred to as a lock-in pixel, where the photogeneratedcharge is distributed between multiple (e.g., 4) wells, each delayed byone quarter of the modulation cycle and lasting half the modulationcycle. The phase shift of the collected signal can be extracted from thecharges collected in each quarter-cycle well. A maximum achievable rangeresolution of some PMDs may be expressed as:

$\sigma_{B} = {\frac{c}{4{\pi \cdot f_{mod}}\sqrt{2}} \cdot \frac{\sqrt{B}}{c_{demod} \cdot A_{sig}}}$

where c is the speed of light, f_(mod) the modulation frequency, B isthe offset which is equal to B=A_(sig)+BG where the first term is thenumber of signal electrons and the second is the number of backgroundelectrons in an integration time, and c_(demod) is the demodulationcontrast.

There may be several drawbacks for PMD devices. For example, PMDstypically use non-CMOS (complementary metal-oxide semiconductor) devicesand may therefore be more expensive than some generic CMOS devices, suchas CMOS Image Sensors. Another shortcoming of PMD devices may beassociated with trade-offs between range and range resolution. Forexample, when four separate and electrically isolated taps are used, thesilicon area required for each pixel increases, thereby reducing themaximal number of pixels on a chip (which area is typically constrainedby the reticle field size in semiconductor manufacturers'photolithography lines) as well as the effective price per pixel. Themaximal range may be determined by maximal phase which can be measured,which is 2π. The frequency can be lowered in order to increase themaximal range. On the other hand, range resolution may be constrained bythe phase error, which may be determined by deviation from a perfectsinusoidal modulation by the emitter and by noise at the four detectortaps, in addition to other amplitude and phase noise between the emitterand detector. Therefore, a higher modulation frequency may be used toincrease or maximize the phase delay for a given distance traversed bythe light pulse. Higher modulation frequencies may require moreexpensive emitter drivers, faster frame-rate detectors, as well ashigher sensitivity, because at higher frequencies, each of the fourphases may spans a shorter amount of time, so less light may beavailable for integration and the signal-to-noise may be reduced. Theissue of dynamic range may be exacerbated by the fact that the signal atthe output of the detector may be lowest for long-range,lower-reflectivity targets which happen to be at a range where the phasedifference between taps may be minimal; whereas the maximal signal mayoccur with short-range, higher-reflectivity targets which happen to beat a distance where the phase difference between taps may be maximal.Moreover, direct sunlight can impede the performance of PMD detectors.While a background subtraction functionality may be integrated into thedetector in some devices, the full-well capacity of each of the tapsshould still accommodate the photon flux due to direct sunlight at thelowest modulation frequency (longest integration), and the Poisson noiseaccompanied by this accumulated charge can degrade the signal-to-noiseratio, especially for distant, weak reflections.

One way to address some of the deficiencies described above is byswitching between frequencies, or tones. However, such switching caneffectively reduce the refresh rate, and may have limited effect becauseachieving ranges of hundreds of meters (as may be desired forapplications such as autonomous vehicles) typically requires very lowfrequencies with very fine phase control. Moreover, the issue of dynamicrange can become severe at long ranges and especially in the presence ofdirect sunlight.

The other category of TOF 3D imagers are direct TOF systems, whichmeasure the distance to targets by measuring the time an optical signaltakes to reach a target and back to the sensor (i.e., the time betweenemission of the optical signal and detection/time of arrival of thereflected optical signal at the sensor). Strobing direct TOF camerastypically use a periodic pulsed light source for illumination and a CCDor CMOS image sensor for detection. The image sensor is activated (or“strobed”) for short windows of time at variable delays with respect tothe light source, thus capturing reflected signals only from specificranges at each frame. Each collected image contains the integratedsignal from all photons in that time (and distance) window. Noinformation may be collected regarding the time of arrival of individualphotons. While such devices can use standard CMOS process technologiesto design the high-speed strobing cameras, their effective refresh rates(RR) may be slow, and may be represented by: RR=(Rangeresolution/Maximal range)×(1/frame time). Therefore, if in one example33 milliseconds (ms) is required to integrate a signal with acceptablesignal to noise ratio (SNR), and 200 m maximal range and 5 cm rangeresolution is desired, the refresh rate is 0.0075 frames per second,which is typically unacceptable. Note that the integration time cannotbe made arbitrarily short due to read noise.

Non-strobing direct TOF 3D imaging systems can use a number of detectionelements, including but not limited to single-element scanning systems,linear-array scanning or rotating systems, and staring or Flash Lidarsystems. Single-element scanning systems, for example utilizingMicro-Electrical Mechanical Systems (MEMS) for beam steering, aretypically constrained by the round-trip time required for a beam toacquire a signal from the farthest target. For example, if the maximaldetectable target is 200 m away, the system may wait 1.3 μsec betweentransmitting a pulse to one direction and the next; otherwise, there maybe ambiguity when receiving a signal as to its originating pulse. Thismay place limitations on the resolution and refresh rate of such asystem. For example, if a resolution of 0.1×0.1 square degrees isdesired across a 120 degree×30 degree field of view with a 200 m maximalrange, the refresh rate of the system would be 2.1 frames/second, whichis typically too slow for many applications (unless short-cuts are takenwhich may result in non-imaged regions or in lower resolutions). Anotherpossible issue with such arrays is the potential for misalignment of theMEMS mirrors, which may result in incomplete and/or inaccurate coverageof the field of view.

Linear arrays for Lidar may use sub-Geiger-mode diode arrays, such asp-i-n diodes and avalanche photodiode (APD)-based arrays. While thephysical operation of each is different—p-i-n diodes may use a widedepletion region to increase quantum efficiency at the expense oftemporal resolution, whereas APDs may use a high electric field toprovide gain at the expense of noise amplification—their operation inthe context of 3D imaging systems is similar. P-i-n diodes and APDs maygenerally be referred to herein as “photodiodes.”

Lidar systems utilizing such photodiodes may operate by emittingperiodic pulsed light. Photons may be absorbed, and in the case of APD'samplified, in the photodiode, generating a current which may beapproximately linearly related to the number of photons. This linearitymay be maintained well with p-i-n diodes but response may deviate fromlinear at high-gain operation of APD's. By measuring the photocurrent ofthe photodiode, weak signals can be measured, and because these devicesdo not integrate charge, they can, in principle operate with highambient light, as long as their noise and the statistical shot noise canbe kept low. This, together with active illumination and spectralfiltering, may allow ambient light imaging. Moreover, by processing theanalog waveform of the generated photocurrent, multiple reflections canbe discriminated and identified.

The direct output of the photodiode is an analog current whichcorresponds to the time-varying photon flux convolved with the temporalresponse of the photodiode and its output impedance. As smallsignal-to-background ratios should be accommodated, digitization of thecurrent may typically take place very close to the sensing junction. AnAnalog to Digital Converter (ADC) may require a relatively large numberof bits to accommodate the high dynamic range and the very fineresolution desired. If there is no redundancy in the array, i.e., if allpixels may record a reflected signal (or “echo”) simultaneously, one ADCcan be allocated for each pixel. This may translate to large die area,so large-scale integration of multi-pixel two-dimensional arrays may belimited to small arrays. Moreover, operating at high gains can limit thebandwidth of the device.

The limited temporal resolution of photodiodes (e.g., 10 ns rise forcertain APDs) may mean that sampling the precise arrival time of theleading edge of the echo can involve a relatively large error. This maylimit the depth resolution of the sensor, which may result in arelatively low spatial resolution, low range resolution system.

Geiger-mode Avalanche diodes may be used in some Lidar systems.Geiger-mode avalanche diodes are p-n diodes that are reverse-biasedbeyond their breakdown voltage. Because a single photon may induce anavalanche, which can in-turn be read out as a binary event whose analoginformation is contained in its time, these devices may not incur readnoise, and may thus be amenable to fast acquisitions with high temporalresolutions. Appropriate circuitry can be designed to provide reliableoperation and to sample the output of Geiger-mode avalanche diodes.

In an imaging Silicon Photomultiplier (SiPM) pixel-array configuration,Geiger-mode avalanche diodes may be organized in clusters of microcells,such that the number of avalanches in a pixel may be used for countingthe number of detected photons in a detection cycle, and appropriatetiming measurement circuitry may be used to detect the time of theseavalanches with respect to a reference time, such as that of an emittedpulse of light. These devices may have a number of deficiencies. Forexample, the maximum number of photons which can be detected for a givenlaser cycle may be limited by the number of microcells in a pixel. Thus,where the diodes are electrically isolated to reduce or preventelectrical or optical cross-talk, a higher number resolution translatesto larger area, which can limit the number of pixels in on-chip arrays.Furthermore, the responsivity of a pixel to the number of avalanches maybe non-linear, which may result in a limited dynamic range, or highererrors in large photon fluxes. If the time of arrival of each photonneeds to be recorded, a large number of analog-to-digital computationsmay be performed, which may result in high area usage on the chip andhigh current consumption. If the capacitance of the diodes is shared,then afterpulsing, which is a correlated noise source in Geiger-modediodes and is thus a source of noise, may increase. Imaging SiPM arraysmay thus be generally used in low-pixel-number arrays, such as a 1×16SiPM. If finer resolution is required, one or more arrays may need to berotated around an axis, resulting in larger and more expensive systems.

Another configuration of Geiger-mode avalanche photodiodes is a SPAD(single photon avalanche detector) array in a Time-CorrelatedSingle-Photon Counting (TCSPC) configuration. For example, as shown inFIG. 26, a controller sends a trigger to a pulsed light source, such asa laser, which in response transmits a pulse at time to. Simultaneously,a Time-to-Digital Converter (TDC) or array thereof, starts measuringtime at time to, using various methods. Some of the photons reflectedfrom target(s) (also referred to herein as echo signals or “echos”) maytrigger one or more avalanches in the SPAD array upon receipt (shown inFIG. 26 with reference to 2 echos, detected at times t1 and t2,respectively). Each avalanche stops its TDC and a digital valuecorresponding to the time elapsed between the laser trigger and thedetection of the avalanche is output and stored in memory. Typically, astatistical distribution of the arrival times of photons in each pixelis produced, from which the 3D position of imaged targets can beinferred (shown in FIG. 26 as object 1 and object 2, based on arrivaltimes t1 and t2 of the echo signals), A similar “Reverse Start-Stop”method may start the time measurement when an avalanche is sensed andend the time measurement at the next laser trigger, which can save powerwhen avalanches are relatively sparse.

In some conventional configurations, a SPAD in an array may be strobedby pre-charging the SPAD beyond its breakdown voltage at a timecorrelated with the firing of an emitter pulse. If a photon is absorbedin the SPAD, it may trigger an avalanche breakdown. This event cantrigger a time measurement in a time-to-digital converter, which in turncan output a digital value corresponding to the arrival time of thedetected photon. A single arrival time carries little informationbecause avalanches may be triggered by ambient light, by thermalemissions within the diode, by a trapped charge being released(afterpulse), and/or via tunneling. Moreover, SPAD devices may have aninherent jitter in their response. Statistical digital processing istypically performed in 3D SPAD-based direct TOF imagers.

Data throughput in such 3D SPAD-based direct TOF imagers is typicallyhigh. A typical acquisition may involve tens to tens of thousands ofphoton detections, depending on the background noise, signal levels,detector jitter, and/or required timing precision. The number of bitsrequired to digitize the time-of-arrival (TOA) may be determined by theratio of range to range resolution. For example, a LIDAR with a range of200 m and range resolution of 5 cm may require 12 bits. If 500acquisitions are required to determine a 3D point in a point cloud, 500time-to-digital conversions may be needed to be performed, and 6 kbitsmay need to be stored for processing. For an example LIDAR system with0.1×0.1 degree resolution and 120 degrees (horizontal) by 30 degrees(vertical) range, 360,000 acquisitions may be performed per imagingcycle. This can require 180 million TDC operations and 2.16 Gbits ofdata. Typical refresh rates for some applications (e.g., autonomousvehicles) may be 30 frames per second. Therefore, a SPAD-based LIDARachieving typical target performance specifications may require 5.4billion TDC operations per second, moving and storing 64.8 Gbit ofinformation and processing 360,000×30=10.8 million histograms persecond.

In addition to such astronomical processing requirements, anarchitecture that uses direct digitization of photons arrival times mayhave area and power requirements that may likewise be incompatible withmobile applications, such as for autonomous vehicles. For example, if aTDC is integrated into each pixel, a large die may only fit 160×128pixels, due for instance to the low fill factor of the pixel (where mostof the area is occupied by control circuitry and the TDC). The TDC andaccompanying circuitry may offer a limited number of bits.

Another deficiency of some existing SPAD arrays is that once a SPAD isdischarged, it remains discharged, or “blind”, for the remainder of thecycle. Direct sunlight is usually taken as 100 k lux. In one example, at940 nm, the direct beam solar irradiance is 0.33 W/m²/nm. At 940 nm,photon energy is 2.1×10⁻¹⁹ J, so 0.33/2.1×10⁻¹⁹=1.6×10¹⁸ photons impingeper m² per second in a 1 nm band. Typical LIDAR filters may have a passband of approximately 20 nm. For a 10 μm diameter SPAD, this translatesto 3.2×10⁹ photons per second. Light takes 400/3×10⁸=1.3 μs to traverse2×200 m. During this time, 3.2×10⁹×1.3×10⁻⁶=4,160 photons on averagewill impinge on the SPAD. As soon as the first photon induces anavalanche, that SPAD will become deactivated. Consequently, under theseconditions, some SPAD 3D cameras may not be operable in direct sunlight.

One method to address high ambient light conditions implements aspatio-temporal correlator. In one example, 4 pixels may be used todigitally detect correlated events, which can be attributed to a pulsedsource rather than to ambient light. Times of arrival of up to 4 SPADsper pixel may be digitized using a fine and coarse TDC, and results maybe stored in a 16-bit in-pixel memory per SPAD. The results may beoffloaded from the chip to be processed in software. The software mayselect coincident arrivals to form a histogram of arrival times perpixel per frame. The histogram may be processed to provide a singlepoint on the point cloud. This scheme may quadruple the area andprocessing power versus generic imagers. By using 4 correlated arrivals,this example system may set limits on emitter power, maximal targetrange and/or target reflectivity, because a single pulse may provide 4detected photons at the detector. Furthermore, the area required for thecircuitry may allow for a limited number of pixels, which may includeonly a small portion of the overall die area. Thus, a high-resolutionimager may be difficult or impossible to implement using this scheme.For example, the data throughput to process a 2×192 pixel array may be320 Mbit/sec, so scaling these 2×192 pixels to the 360,000 pixelsmentioned above for a staring LIDAR system may be unrealistic.

SUMMARY

According to some embodiments of the present disclosure, a LightDetection And Ranging (LIDAR) apparatus includes a pulsed light sourceconfigured to emit optical signals; a detector array comprisingsingle-photon detectors that are configured to output respectivedetection signals indicating respective times of arrival of a pluralityof photons incident thereon, where the photons comprise signal photonshaving wavelengths corresponding to the optical signals from the pulsedlight source and background photons having wavelengths corresponding toat least one other source of light (e.g., ambient light); and processingcircuitry configured to receive the respective detection signals outputfrom the single-photon detectors. The processing circuitry includes oneor more of a recharge circuit configured to activate and deactivatesubsets of the single photon detectors for respective strobe windowsbetween pulses of the optical signals and at respective delays thatdiffer with respect to the pulses, responsive to respective strobingsignals; a correlator circuit configured to output respectivecorrelation signals representing detection of one or more of the photonswhose respective time of arrival is within a predetermined correlationtime relative to at least one other of the photons; and a timeprocessing circuit comprising a counter circuit configured to incrementa count value responsive to the respective correlation signals ordetection signals and a time integrator circuit configured to generatean integrated time value with respect to a reference timing signal basedon the respective times of arrival indicated by the respectivecorrelation signals or detection signals, where a ratio of theintegrated time value to the count value indicates an average time ofarrival of the photons.

In some embodiments, a tunable optical filter element may be arranged topass or transmit the photons that are incident on the detector array.The tunable optical filter element may have a transmission band that isconfigured to vary based on a spectrum of optical signals output from apulsed light source and/or a temperature of the pulsed light source.

In some embodiments, the processing circuitry may further include afirst channel that is configured to provide output values responsive toa first subset of the detection signals indicating the respective timesof arrival of the plurality of photons including the signal photons andthe background photons; a second channel that is configured to providereference values responsive to a second subset of the detection signalsindicating the respective times of arrival of the background photons butnot the signal photons; and a control circuit that is configured tocalculate an estimate of the average time of arrival of the photonsbased on a mathematical relationship between the output values and thereference values.

In some embodiments, the processing circuitry may be integrated on-chipwith the detector array.

In some embodiments, the single-photon detectors may be single-photonavalanche detectors (SPADs).

In some embodiments, a control circuit may be configured to generate therespective strobing signals and/or calculate the average time of arrivalof the photons.

In some embodiments, the control circuit may be integrated on-chip withthe detector array.

According to some embodiments of the present disclosure, a LightDetection And Ranging (LIDAR) measurement device includes a detectorarray comprising single-photon detectors that are configured to outputrespective detection signals indicating respective times of arrival ofphotons incident thereon, where the photons comprise signal photonshaving wavelengths corresponding to optical signals output from a pulsedlight source; and processing circuitry comprising a recharge circuitthat is configured to activate and deactivate subsets of the singlephoton detectors for respective strobe windows between pulses of theoptical signals and at respective delays that differ with respect to thepulses, responsive to respective strobing signals.

In some embodiments, durations of the respective strobe windows may bethe same.

In some embodiments, durations of the respective strobe windows maydiffer.

In some embodiments, a time between the pulses of the optical signalsmay correspond to a distance range, and the durations of the respectivestrobe windows may differ according to sub-ranges of the distance range.

In some embodiments, the durations of the respective strobe windowscorresponding to closer sub-ranges of the distance range may be greaterthan the durations of the respective strobe windows corresponding tofarther sub-ranges of the distance range.

In some embodiments, the recharge circuit may be configured to activateand deactivate the subsets of the single photon detectors for therespective strobe windows responsive to the respective strobing signalsbased on relative positions of the subsets of the single photondetectors in the detector array.

In some embodiments, the relative positions may correspond to differentazimuths and altitudes of an operating environment relative to anorientation of the detector array.

In some embodiments, the recharge circuit may be configured todynamically adjust the durations of the respective strobe windowsresponsive to the respective strobing signals.

In some embodiments, the recharge circuit may be configured todynamically adjust the durations of the respective strobe windowsresponsive to the respective strobing signals so as to alter boundariesof the sub-ranges corresponding to the respective strobe windows, orbased on a brightness of a target indicated by previous detectionsignals.

According to some embodiments of the present disclosure, a LightDetection And Ranging (LIDAR) measurement device includes a detectorarray comprising single-photon detectors that are configured to outputrespective detection signals indicating respective times of arrival of aplurality of photons incident thereon, where the photons comprise signalphotons having wavelengths corresponding to optical signals output froman emission source and background photons having wavelengthscorresponding to at least one other light source; and processingcircuitry configured to receive the respective detection signals outputfrom the single-photon detectors. The processing circuitry includes atime processing circuit comprising a counter circuit configured toincrement a count value responsive to the respective detection signals,and a time integrator circuit configured to generate an integrated timevalue with respect to a reference timing signal based on the respectivetimes of arrival indicated by the respective detection signals, where aratio of the integrated time value to the count value indicates anaverage time of arrival of the photons.

In some embodiments, the processing circuitry may further include arecharge circuit that is configured to activate and deactivate subsetsof the single photon detectors for respective strobe windows betweenpulses of the optical signals and at respective delays that differ withrespect to the pulses, responsive to respective strobing signals.

In some embodiments, the processing circuitry may further include acorrelator circuit that is configured to receive the respectivedetection signals and output respective correlation signals representingdetection of one or more of the photons whose respective time of arrivalis within a predetermined correlation time relative to at least oneother of the photons. The counter circuit may be configured to incrementthe count value responsive to a subset of the respective detectionsignals comprising the correlation signals, and the time integratorcircuit may be configured to integrate the respective times of arrivalindicated by the subset of the respective detection signals comprisingthe correlation signals.

In some embodiments, a tunable optical filter element may be arranged tooutput the photons that are incident on the detector array. The tunableoptical filter element may have a transmission band that is configuredto vary based on a spectrum of the optical signals and/or temperature ofthe emission source.

In some embodiments, the time processing circuit may include a firstchannel that is configured to provide the count value and the integratedtime value responsive to a first subset of the detection signalsindicating the respective times of arrival of the plurality of photonsincluding the signal photons and the background photons, and a secondchannel that is configured to provide a reference count value and areference integrated time value responsive to a second subset of thedetection signals indicating the respective times of arrival of thebackground photons but not the signal photons. A control circuit may beconfigured to calculate an estimate of the average time of arrival ofthe photons based on relationships between the integrated time value andthe reference integrated time value, and between the count value and areference count value.

In some embodiments, the counter circuit may include a countingcapacitor configured to accumulate charge responsive to each of therespective detection signals and output a voltage corresponding to thecount value; and/or the time integrator circuit may include anintegrating capacitor configured to accumulate charge responsive to therespective detection signals and output a voltage corresponding to theintegrated time value.

According to some embodiments of the present disclosure, a LightDetection And Ranging (LIDAR) measurement device includes a detectorarray comprising single-photon detectors that are configured to outputrespective detection signals indicating respective times of arrival of aplurality of photons incident thereon; and processing circuitryconfigured to receive the respective detection signals output from thesingle-photon detectors. The processing circuitry includes a correlatorcircuit that is configured to output respective correlation signalsrepresenting detection of one or more of the photons whose respectivetime of arrival is within a predetermined correlation time relative toat least one other of the photons.

In some embodiments, the correlator circuit may be configured to outputthe correlation signals independent of stored data indicating therespective times of arrival based on the detection signals, in someembodiments.

In some embodiments, the correlator circuit may be configured to outputthe correlation signals without storing the respective times of arrivalin one or more histograms.

In some embodiments, the predetermined correlation time may be relativeto a leading edge of the respective detection signal indicating therespective time of arrival for the one or more of the photons.

In some embodiments, the predetermined correlation time may correspondto a pulse width of optical signals output from a pulsed light source.

In some embodiments, the correlator circuit may include respectivebuffer elements that are configured to delay the respective detectionsignals by the predetermined correlation time and output respectivepulsed signals having pulse widths corresponding to the predeterminedcorrelation time; and logic circuits that are configured output thecorrelation signals when the pulse widths of at least two of therespective pulsed signals overlap in time.

In some embodiments, the processing circuitry may further include a timeprocessing circuit comprising a counter circuit configured to incrementa count value responsive to each of the correlation signals, and a timeintegrator circuit configured to generate an integrated time value basedon the respective times of arrival corresponding to the correlationsignals, where a ratio of the integrated time value to the count valueindicates an estimated average time of arrival of the photons.

In some embodiments, the processing circuitry may be configured tobypass the correlator circuit and provide the respective detectionsignals to the time processing circuit based on the respective detectionsignals relative to a predetermined threshold.

In some embodiments, the time processing circuit may include a firstchannel that is configured to provide the count value and the integratedtime value responsive to the correlation signals, and a second channelthat that is configured to provide a reference count value and areference integrated time value responsive to respective detectionsignals corresponding to photons whose respective times of arrival areoutside the predetermined correlation time relative to one another.

In some embodiments, the correlator circuit may be configured toincrease or decrease the predetermined correlation time when therespective detection signals corresponding to photons whose respectivetimes of arrival are outside the predetermined correlation time relativeto one another are below a threshold.

In some embodiments, the processing circuitry may further include arecharge circuit that is configured to activate and deactivate subsetsof the single photon detectors for respective strobe windows betweenpulses of optical signals output from a pulsed light source and atrespective delays that differ with respect to the pulses, responsive torespective strobing signals.

In some embodiments, a tunable optical filter element may be arranged tooutput the photons that are incident on the detector array, the tunableoptical filter element having a transmission band that is configured tovary based on a spectrum of optical signals output from a pulsed lightsource and/or a temperature of the pulsed light source.

According to some embodiments of the present disclosure, a LightDetection And Ranging (LIDAR) measurement device includes a tunableoptical filter element having a transmission band that is configured tovary based on a spectrum of optical signals output from an emissionsource and/or a temperature of the emission source; and a detector arrayarranged to receive output light transmitted through the optical filterelement, the detector array configured to output respective detectionsignals indicating respective times of arrival of a plurality of photonsincident thereon.

In some embodiments, at least one actuator may be configured to alter atilt angle of the tunable optical filter element relative to a referenceangle (e.g., an angle of incidence of light thereon). The tilt angle maybe continuously variable over a predetermined angular range, or may bevariable among a plurality of discrete tilt angles, and the transmissionband may vary based on the tilt angle.

In some embodiments, an impedance measurement circuit may be configuredto measure respective impedances at respective regions of the tunableoptical filter element, and a driving circuit may be coupled to theimpedance measurement circuit and configured to control the at least oneactuator to alter the tilt angle based on the respective impedances.

In some embodiments, a temperature of the tunable optical filter elementmay be configured to vary with a temperature of the emission source.

In some embodiments, the tunable optical filter element may be thermallycoupled to the emission source, comprises a same material as theemission source, and/or is included in a temperature-controlled housing.

According to some embodiments of the present disclosure, a LightDetection And Ranging (LIDAR) measurement device includes a detectorarray configured to output respective detection signals indicatingrespective times of arrival of photons incident thereon, wherein thephotons comprise signal photons having wavelengths corresponding tolight output of an emission source and background photons havingwavelengths corresponding to at least one other light source; andprocessing circuitry configured to receive the respective detectionsignals output from the single-photon detectors. The processingcircuitry includes a first channel that is configured to provide outputvalues responsive to a first subset of the detection signals indicatingthe respective times of arrival of the plurality of photons includingthe signal photons and the background photons; and a second channel thatis configured to provide reference values responsive to a second subsetof the detection signals indicating the respective times of arrival ofthe background photons without the signal photons. A control circuit isconfigured to calculate an estimate of the average time of arrival ofthe photons based on a mathematical relationship between the outputvalues and the reference values.

In some embodiments, the control circuit may be configured tosequentially operate one or more single-photon detectors of the detectorarray to provide the first and second subsets of the detection signals.

In some embodiments, the control circuit may be configured tosequentially operate the one or more of the single-photon detectors toprovide the second subset in coordination with deactivation of theemission source.

In some embodiments, the control circuit may be configured to operateone or more single-photon detectors of the detector array to provide thesecond subset in parallel with the first subset. The one or more of thesingle photon detectors may comprise an optical filter thereon having atransmission band that is configured to prevent passage of the signalphotons to the one or more of the single photon detectors.

In some embodiments, the processing circuitry may further include acorrelator circuit that is configured to receive the respectivedetection signals and output respective correlation signals representingdetection of one or more of the photons whose respective times ofarrival are within a predetermined correlation time relative to oneanother as the first subset.

In some embodiments, the correlator circuit may be configured toincrease or decrease the predetermined correlation time when the secondsubset of the detection signals indicate that light from the at leastone other light source is below a threshold.

According to some embodiments, a Light Detection And Ranging (LIDAR)imaging device includes an array of single-photon detectors (e.g.,SPADs) that are configured to output respective detection signalsindicating respective times of arrival of photons incident thereon, andan array of infrared detectors and/or CMOS image sensors integrated inthe array of single photon detectors.

In some embodiments, the single photon detectors may have a concentricarrangement (e.g., with a central diode surrounded by one or morering-shaped diodes), and may share one or more electrical connections ormay have their own electrical connections

In some embodiments, the single photon detectors may have a stackedarrangement (e.g., with one or more diodes arranged under a firstdiode), and may share one or more electrical connections or may havetheir own electrical connections.

In some embodiments, an array of capacitors may be provided on theimaging device (e.g., on a same substrate with the array stackedthereon) so as to allow charge distribution and fast recharging of thesingle-photon detectors of the array.

Other devices, apparatus, and/or methods according to some embodimentswill become apparent to one with skill in the art upon review of thefollowing drawings and detailed description. It is intended that allsuch additional embodiments, in addition to any and all combinations ofthe above embodiments, be included within this description, be withinthe scope of the invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating example components of a time offlight measurement system or circuit in a LIDAR application inaccordance with some embodiments described herein.

FIG. 2 is a block diagram illustrating example components of a time offlight measurement system or circuit in a LIDAR application inaccordance with further embodiments described herein.

FIG. 3 is a block diagram illustrating example components of an imagingsystem including a tunable optical filter in a LIDAR application inaccordance with some embodiments described herein.

FIGS. 4A-4D are graphs illustrating characteristics of tunable opticalfilters that may be used in in a LIDAR application in accordance withsome embodiments described herein.

FIGS. 5A-5C are diagrams illustrating SPAD-based 3D imaging systems andassociated operations that may be performed in accordance with someembodiments described herein.

FIG. 6 is a block diagram illustrating operations for in-pixel datareduction in accordance with some embodiments described herein.

FIG. 7 is a diagram illustrating relationships between image frames,subframes, laser cycles, and strobe windows in accordance with someembodiments described herein.

FIGS. 8A and 8B are diagrams illustrating examples of range strobing inaccordance with some embodiments described herein.

FIG. 9 is a block diagram illustrating an example LIDAR measurementdevice in accordance with some embodiments described herein.

FIG. 10 is a block diagram illustrating an example counting circuit inaccordance with some embodiments described herein.

FIGS. 11A and 11B are block diagrams illustrating examples of timeprocessing circuits in accordance with some embodiments describedherein.

FIGS. 12A and 12B are plan and cross-sectional views, respectively,illustrating an example detector pixel including multiple detectorelements in accordance with some embodiments described herein.

FIG. 13 is a cross-sectional view of an example detector pixel includingmultiple detector elements in accordance with further embodimentsdescribed herein.

FIG. 14 is a block diagram illustrating an example saturation controlcircuit in accordance with some embodiments described herein.

FIG. 15 is a block diagram illustrating an example pulse time correlator(PTC) circuit in accordance with some embodiments described herein.

FIGS. 16A-16C are timing diagrams illustrating example operations of thePTC circuit of FIG. 15 in accordance with some embodiments describedherein.

FIG. 17 is a block diagram illustrating an example detector sub-array inaccordance with some embodiments described herein.

FIG. 18A is a graph illustrating characteristics of tunable opticalfilters that may be used in in a LIDAR application in accordance withsome embodiments described herein

FIG. 18B is a block diagram illustrating an example integrated visibleTOF-IR image sensor device in accordance with some embodiments of thepresent disclosure.

FIG. 19 is a block diagram illustrating an example analog timeprocessing circuit in accordance with some embodiments described herein.

FIGS. 20A and 20B illustrate example operations for distinguishingbetween signal and background photons in accordance with someembodiments described herein.

FIG. 21 is a graph illustrating effects of noise in example operationsfor distinguishing between signal and background photons in accordancewith some embodiments described herein.

FIG. 22 is a block diagram illustrating example operation of acorrelator circuit in accordance with some embodiments described herein.

FIG. 23 is a block diagram illustrating an example dual pixel elementfor background photon correction operations in accordance with someembodiments described herein.

FIG. 24A and FIG. 24B are graphs illustrating operations for phaseshifting to correct for distributions of detected photons that may spantwo subframes in accordance with some embodiments described herein.

FIGS. 25A and 25B are block diagrams illustrating examples of tunableoptical filter configurations in accordance with some embodimentsdescribed herein.

FIG. 26 is a diagram illustrating operational principles of SPADs thatmay be used in conjunction with LIDAR systems and measurement circuitsin accordance with some embodiments described herein.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure are directed to light-basedranging measurement systems (such as LIDAR) and related methods ofoperation that are configured to reduce the quantity of incoming photonsthat are measured and/or stored as data in memory. Some embodimentsdescribed herein provide methods, systems, and devices includingelectronic circuits that provide LIDAR systems including one or moreemitter elements (including semiconductor lasers, such as surface- oredge-emitting laser diodes; generally referred to herein as emitters)and one or more light detector elements (including semiconductorphotodetectors, such as photodiodes, including avalanche photodiodes andsingle-photon avalanche detectors; generally referred to herein asdetectors). In some embodiments, photons are selectively captured ordetected by the detectors based on a time correlation between theirrespective times of arrival relative to one another, which can reducethe quantity of incoming photons that are measured and processed. Forexample, based on recognition that photons from a pulsed laser andreflected by a target may arrive in a relatively narrow window of time,embodiments described herein can thereby selectively capture these“correlated” photons while rejecting “uncorrelated” photons, such asphotons from ambient light sources (e.g., the sun). In some embodiments,a counter circuit, such as an analog counter, generates count valuesignals representative of the photons that fall within the timecorrelation window, providing in-pixel averaging without digitizing andstoring histograms or other data representative of the captured photons.Thus, data throughput can be significantly reduced.

Effects of ambient light can be further reduced by strobing range gatesnon-linearly and/or by spectral filtering of the light output by theemitter array and/or the light detected at the detector array. Inparticular, further embodiments may include tunable spectral filters(e.g., varying with emitter or detector array temperature and/or emitterarray spectral output), non-linear data strobing (e.g., varying withtime of flight) to further reduce ambient light photon counts. Detectionand subtraction of uncorrelated or “background” photons may also beimplemented. Minimal or reduced off-chip processing may be required,thereby lowering overall system cost. It will be understood thatdiscussion herein with reference to ambient light or light sources maylikewise apply to light from sources other than the pulsed laser oremission source of the LIDAR system of the present disclosure.

That is, some embodiments of the present disclosure may include acombination of in-pixel counting and averaging with a time correlatorand, in some further embodiments, non-linear strobing of the detectorarray, background light subtraction, and/or tunable spectral filtering.In some embodiments, avalanche photodiodes, such as SPAD-based arrays,may be used as a photon capture mechanism. Some embodiments can thusprovide long-range staring SPAD-based LIDAR systems operating in directsunlight conditions. Additional features of the present disclosure,including any and all combinations of such features, will becomeapparent from the following description and appended claims, taken inconjunction with the accompanying drawings.

FIG. 1 Illustrates example components of a time of flight measurementsystem or circuit 100 in a LIDAR application in accordance with someembodiments described herein. The circuit may include a control circuit105, a timing circuit 106, and an array of single-photon detectors 110(for example, a SPAD array). The timing circuit 106 may generate andoutput strobing signals that control the timing of the single-photondetectors of the array 110. An emitter array 115 emits a radiation pulse(for example, through a diffuser or optical filter 114) at a timecontrolled by a timing generator or driver circuit 116.

In some embodiments, each of the emitter elements in the emitter array115 may be connected to and controlled by a respective driver circuit116. In other embodiments, respective groups of emitter elements in theemitter array 115 (e.g., emitter elements in spatial proximity to eachother), may be connected to a same driver circuit 116. The drivercircuit 116 may include one or more driver transistors, which areconfigured to control the timing and amplitude of the optical emissionsignal. The timing circuit 106 may likewise control the timing andgain/sensitivity of the detector array 110. In some embodiments, thetiming circuit 106 and/or the driver circuit 116 may be included in thecontrol circuit 105.

Optical signals emitted from one or more of the emitters of the emitterarray 115 impinges on and is reflected by one or more targets 150, andthe reflected light is detected as an optical signal (also referred toherein as an echo signals or echo) by one or more of the detectors ofthe detector array 110 (e.g., via one or more lenses 112), convertedinto an electrical signal representation, and processed (e.g., based ontime of flight) to define a 3-D point cloud representation 170 of thefield of view. More particularly, the detector array 110 generatesrespective detection signals indicating the respective times of arrivalof photons in the reflected optical signal, and outputs the respectivedetection signals to the control circuit 105. In some embodiments, thecontrol circuit 105 may include a pixel processor that measures the timeof flight of the illumination pulse over the journey from the emitterarray 110 to a target 150 and back to the detector array 110 (i.e., thetime between emission of the optical signal by the emitter array 115 andthe time of arrival of the reflected optical signal or echo at thedetector array 110, as indicated by the respective detection signals)and calculates the distance to the target 150. Operations of LIDARsystems in accordance with embodiments of the present invention asdescribed herein may be performed by one or more processors orcontrollers, such as the control circuit 105 of FIG. 1. Portions or anentirety of the control circuits described herein may be integrated inthe detector array 110 in some embodiments.

In particular embodiments, the emitter array 115 may include a pulsedlight source, such as an LED, laser, VCSEL, or arrays thereof. The totaloptical power output of the light source may be selected to generate asignal-to-noise ratio of the echo signal from the farthest, leastreflective target at the brightest background illumination conditionsthat can be detected in accordance with embodiments described herein.The emitted light can have a relatively narrow bandwidth. In somenon-limiting examples, the total emitter peak power may be 0.01, 0.1, 1,5, 10, 25, 40, 60, or 65 kW with a peak wavelength of 940 nm with anemission bandwidth of about 0.1, 0.5, 1, 3, 5, 10, or 20 nm FWHM (fullwidth at half maximum).

In some embodiments, the emitter array 115 may be an array of VCSELs.The typical emission wavelength spectra of VCSELs on a wafer may bebroader than typically desirable in LIDAR applications. For example,peak emission wavelength may vary by about 10 or 20 nm across a wafer.Also, there may be a high spatial correlation between peak emissionwavelength of VCSELs on a wafer. In other words, VCSEL devices which arein close proximity on a wafer typically have close emission spectra andthese spectra can be measured, e.g., by optical pumping, before dicingor singulation of the VCSEL devices.

Some embodiments described herein may be directed to LIDAR systems that,for example, have particular application for use on a vehicle, such asan autonomous vehicle. The following discussion of embodiments directedto a LIDAR system for autonomous vehicles is merely exemplary in nature,and is in no way intended to limit the disclosure or its applications oruses. For example, while some embodiments of LIDAR systems describedherein may have particular application for use on a vehicle, as will beappreciated by those skilled in the art, the LIDAR systems describedherein may have other applications, including but not limited to roboticsystems, aerial vehicles and warehouse navigation equipment.

It may be desirable for a LIDAR system to include all solid-statecomponents and require no mechanical scanning, reducing cost andincreasing robustness. Such a LIDAR system may have a range of severalhundred meters, e.g., 150, 200, 250 or 300 meters, may be operableduring daytime and nighttime lighting conditions, even with directsunlight (100 k lux), and may provide a fine range resolution, e.g., 3,5, 7, 10, or 15 cm. Some or all regions of the field of view may berefreshed with desired frequency, e.g., 10, 20, 30, 40, or 50 frames persecond. The angular field of view may be relatively wide for vehicularapplications, e.g., 120 degrees horizontal×30 degrees vertical fieldwith 0.1 degree resolution. The wavelength, output power, and emissioncharacteristics of the emitter(s) may not cause eye damage. The LIDARsystem may operate at a relatively wide temperature range for outdooroperating conditions, e.g. −40 degrees Celsius (C) to 105 degrees C.ambient, have a small form factor and be cost-effective.

Such an example LIDAR system 200, as shown in FIG. 2, includes anarrow-emission-band VCSEL array 215 as the emitter array along withbeam shaping emitter optics 214 to emit optical signals that may cover adesired field of view (FOV). For example, the VCSEL array 215 may beprinted onto a flexible substrate and interconnected using an overlaidmetal-dielectric stencil to ensure substantially simultaneous firing ofthe array of VCSELs. The interconnected array and substrate may bemounted on a thermally conductive second substrate and enclosed in anenclosure. One or more emission lenses in (or, alternatively, out of)the enclosure may diffuse the VCSEL emissions to form an emission coneto illuminate a desired area.

In the example LIDAR system 200, the detector array is implemented as aSPAD array 210. As described in greater detail herein, the SPAD array210 may include a plurality of pixels, each of which contains two ormore SPADs, a time correlator, an analog counter, and/or a timeaccumulator. The correlator, counter, and/or accumulator may beintegrated on-chip (e.g., stacked below on a same substrate) with thearray of SPAD detectors 210. During each imaging frame, a controller 205drives the VCSEL array 215 to illuminate a portion or entirety of thefield of view using optical signals 217 including a train of pulses. Animaging filter 212 passes most or substantially all the arriving echoVCSEL photons 218, yet rejects (a majority of) ambient photons. TheSPADs of the array 210 may be discharged when the VCSELs of the array215 fire, and may be (fully) recharged a short time after the emissionof the optical pulse. Some embodiments described herein implement a timecorrelator, such that only pairs of (or more than two) avalanchesdetected within a pre-determined time are measured. In some embodiments,a measurement may include the addition of a fixed first charge(indicating a count value) onto a counting capacitor, as well as theaddition of a second charge (which is a function of the arrival time)onto a time integrator. At the end of a frame, a circuit (illustrated asincluding a readout integrated circuit (ROIC) 208 and a GPU/point cloudprocessor 209) calculates the ratio of integrated time to number ofarrivals, which is an estimate of the average time of arrival of photonsfor the pixel. This estimate is based on calculation of a “center” ofthe integrated time distribution, and is also referred to herein ascenter of mass estimation. The processor 209 collects the point clouddata from the imager module (referred to herein as including thedetector array and accompanying processing circuitry), generating a 3Dpoint cloud.

As shown in the example of FIG. 2, a temperature monitor 213 measuresthe temperature of the VCSEL array 215 and outputs electrical signalsindicating the temperature. The VCSEL array 215 may be configured toemit light as optical signals 217 including a train of pulses at awavelength which is weakly absorbed by the atmosphere, for example, at awavelength of about 940 nm, and simultaneously illuminate a relativelywide field of view, e.g., a region of 120° horizontal by 30° vertical.Some of the emission light 217 hits a target 250 and some of this lightis reflected to provide echo signals 218 onto the detector/SPAD array210, which may be arranged in proximity to the emitter/VCSEL array 215.

Various optical spectral imaging filters can be used as the imagingfilter 212 to block some of the ambient light and transmit some, most,or all of the emission light 217 output from the emitter array 215 ontothe detector array 210. Some spectral filters may utilize an absorbingmaterial, and some may use a stack of dielectric materials (dielectricfilters). Others may use a cavity, such as a Fabry-Perot Interferometer,to selectively transmit wavelengths of light corresponding to theoptical emission 217 of the LIDAR system 200 while blocking much of theambient light. The transmission band of such filters can be tuned, forexample, to transmit light over a bandwidth of about 20 nm or 10 nm or 1nm or 0.1 nm.

In FIG. 2, the imaging filter is implemented as a tunable optical filterelement 212 (also referred to herein as a tunable filter) that isconfigured to transmit the received reflected light 218 (e.g.,substantially all of the received reflected light) from the target 250,but reject or block transmission of at least some portion of ambientlight and/or light from other LIDAR emitters, that is, light that isoutside of the wavelength range of the emission light 217 from the VCSELarray 215. The transmission band (or “pass band”) of the tunable filter212 may be relatively narrow, e.g., 0.1, 0.5, 0.8, 1, 1.5, 3, 5, 7, 10,or 20 nm FWHM. In some embodiments, the transmission band of the tunablefilter 212 may be controlled or varied based on changes in thetemperature of the emitter array 215 (and/or resulting changes in thespectral output 217 thereof), for example, in response to the electricalsignals from the thermal sensor/temperature monitor 213.

In some embodiments, the wafer containing the VCSEL array 215 may beprobed and an emission wavelength map may be produced. In someembodiments, a transfer process, such as a micro-transfer printingprocess, transfers groups of VCSELs to define the VCSEL array 215 with arelatively narrow spectral distribution; for example, VCSELs whoseemission maxima are with a 1-nm band of one another may be transferredto a single substrate. In some embodiments, wafer mapping may not beperformed, and narrow spectral spread may be ensured based on thelocalization of transferred VCSELs. In some embodiments, an interconnectlayer may be applied on or above the array 215 of VCSEL elements suchthat the time delay from one or more drivers to each of the VCSELelements of the array 215 is relatively small. For example and inparticular embodiments, a single driver (such as the driver circuit 116of FIG. 1) drives an interconnect “tree” whereby the interconnect lengthand the RC delay from the driver to some or all VCSEL elements of thearray 215 may be less than about 250 picoseconds (ps), or 500 ps, or 750ps, or 1 ns. One or more lenses (such as the lens 112) may be arrangedto homogenize the emission for the VCSELs and shape a beam to illuminatea desired region with desired uniformity.

As shown in FIG. 2, the tunable filter 212 is a narrow-band imagingfilter arranged to filter the optical signals 218 reflected from atarget 250 before detection by the SPAD array 210. The tunable filter212 is controlled by the measured temperature of the VCSEL array 215 (asindicated by output from the temperature monitor 213) and transmits anarrow spectral band corresponding to (substantially all of) the emittedVCSEL energy to the SPAD array 210, while rejecting ambient light. Thetunable filter 212 may include tunable Fabry Perot filters,acousto-optical filters, and liquid crystal tunable filters. Some ofthese filters may suffer from broad transmission bands, slow switchingbetween spectral bands, low light throughput, low out-of-band rejectionratio, and/or high price. In some embodiments, collection optics (suchas the lens 112 of FIG. 1) may be used to focus reflected light 218 ontothe SPAD array 210 such that each SPAD pixel images a 0.1°×0.1° cone.

FIG. 3 shows one example of an imaging system 300 including a tunablefilter 312 according to some embodiments. Collection optics 318, whichmay include one or more optical elements, collimate light from apredefined acceptance cone, e.g., 120° horizontal×30° vertical withrespect to the optical axis of the system 300. Spatial filters 311,which may be implemented in various forms such as a barrel, are arrangedso as to allow only a relatively narrow set of ray angles to betransmitted through them. For example, as shown in FIG. 3, Ray A (whichis at least partially collimated) passes through the 2-pupil spatialfilter 311, while Ray B (which is incident at an angle that is greaterthan or outside of the angular range of the acceptance cone) is blocked.A tunable filter 312 with desired finesse is attached to a stationarysubstrate 345 by actuators 344 (such as piezoelectric actuators, whosedriving voltage may be set by a thermal sensor attached to the emissionVCSEL array). Focusing optics 316 focuses the spatially- andspectrally-filtered light onto an optical sensor or detector array 310,which may be a SPAD array in some embodiments.

As shown in greater detail in the example of FIG. 3, the tunable filter312 is arranged in front of the detector array 310 to substantiallyblock ambient light and/or other light that is not output from theemitter array (e.g., 115, 215). In particular embodiments the tunablefilter 312 may be a high-finesse tunable Fabry-Perot Interferometer, ahigh-finesse liquid-crystal tunable filter, a tunable acousto-opticaltunable filter, or a dielectric filter, each of which may be mounted onone or more actuators 344 and (optionally) placed behind a spatialfilter (such as defined by the pupils 311) to direct collimated light ata controlled angle with respect to the filter 312. The actuator(s) 4 maybe configured to tilt the tunable filter 312 at a desired tilt angle inresponse to an actuator control signal. The actuators 344 may beoperable to alter the tilt angle of the tunable filter 312 over acontinuously variable angular range, or among a set of discrete tiltangles corresponding to respective actuator positions. The one or morepupils 311 may be coaxial with the desired propagation of incominglight.

Spectral filters may be used for rejecting sunlight but allowing LIDARreflection light to reach the detectors. The transmission bands of suchfilters are typically 1-20 nm wide. A transmission band of an examplenarrow band LIDAR filter is shown in FIG. 4A. The transmission bandcenter wavelength is a function of the incident light ray angle withrespect to its optical axis:

λ(θ)=λ₀√{square root over (1−(sin θ/n _(eff))²)}

and is shown by way of example in FIG. 4B for n_(eff)=1.8. The centralemission wavelength of VCSELs may vary approximately linearly withtemperature at a rate of approximately 0.08° C./nm. An example of anemission spectrum of an emitter array (e.g., the VCSEL array 215) as afunction of temperature is shown in FIG. 4C. In some embodiments, thetemperature of the emitter array is monitored (e.g., by temperaturemonitor 213), and a microcontroller generates a tilt signal in responseto the temperature of the emitter array to control actuator elements 4(e.g., piezoelectric elements) attached to the narrowband filter 312such that the transmission wavelength of the filter 312 tracks thenarrow emission band of the emitter array, for example, based on thetemperature-angle relationship shown in FIG. 4D.

In the embodiment of FIG. 3, the tunable filter 312 is placed in theFourier plane of the optical system 10. In some embodiments, the tunablefilter 312 is placed in the focal plane, such that the numericalaperture of the system 10 is sufficiently low to provide a relativelynarrow pass band (e.g., 0.1, 0.5, 0.8, 1, 1.5, 3, 5, 7, 10, or 20 nmFWHM), while maintaining consistency between the numerical aperture andthe diameter of the aperture. In some embodiments, the imaging system 10may be configured to maintain a temperature of the environment of thetunable filter 312 within a predetermined temperature range.

FIGS. 5A, 5B, and 5C illustrate some SPAD-based 3D imaging systems toaid understanding of embodiments described herein. In particular, dataflow of some 3D direct TOF SPAD arrays is shown in FIG. 5A. For example,for emitter array light output at about 940 nm, direct beam solarirradiance may be about 0.33 W/m²/nm. Photon energy may be approximately2.1e-19 J, so 0.33/2.1e-19=1.6e18 photons may impinge per m² per secondin a 1 nm pass band, and 3.2e19 photons may impinge in a 20 nm passband. For a 10 μm diameter SPAD, this translates to 3.2e9 photons persecond. Light takes 400/3e8=1.3 μs to traverse 2×200 m. During thistime, 13.2e9×1.3e-6=416 photons on average will impinge on the SPADevery laser cycle. In some architectures, the SPAD array is rechargedonce per cycle so upon an avalanche, a SPAD becomes unresponsive toadditional photons, and thus, the imaging system cannot function.

If filtering is implemented to reduce average photon detection to 1 perlaser cycle, detection of each avalanche (at block 510) may require atime digitization (at block 515) by a time to digital converter (TDC)circuit. There may be 1/1.3e-6=800,000 such conversions per pixel persecond, and 800,000×360,000=288 billion time to digital conversions forthe whole array per second. The TDC power consumption may be about 22.5nW, so the detector array may require 6.5 kW, which may be unattainablein a small form factor autonomous vehicle LIDAR. Furthermore, todigitize with 1 nsec resolution (15 cm) and 1.3 μsec range may require11 bits of resolution. An array of compact TDC's with this resolutionmay occupy 4 full-photolithography reticle dies in a 28 nm processtechnology, even before considering how to route all the signalsat-speed and with sub-ns jitter, which may be impractical in staring 3Dlong-range LIDAR systems.

Embodiments of the present disclosure can reduce the number of requiredtime to digital conversions based upon recognition that photonsoriginating from the solar background arrive uncorrelated, whereasphotons from a target illuminated by a pulsed light source (e.g., apulsed laser) have a higher likelihood of being detected in groups of 2or more in a narrow time correlation window (for example a time windowcorresponding to the pulse width), also referred to herein as apredetermined correlation time. Such embodiments, some of which may beimplemented in a quasi-linear SPAD array (versus an area, staringdetector), are illustrated in FIG. 5B. In particular, as shown in FIG.5B, after detection of photon arrival (at block 510), a time correlatorcircuit outputs signals indicating detection of correlated photon pairshaving respective times of arrival that fall within the predeterminedcorrelation time relative to one another (at block 525). This can reducethe number of time to digital conversions, but a TOA histogram wouldstill be generated for each pixel during each acquisition frame, whichinvolves digitization of up to several hundred TOAs per pixels per frame(block 515), storage of the digitized data in a memory (block 530), andgeneration of a histogram (at block 540). In some instances, 10.8million histograms per second may be needed, which may be beyond thecomputational power of cost-effective mobile platforms.

FIG. 5C illustrates an example time-of-arrival histogram (as generatedat block 540), with 100 k lux background and a target 55 m from theemitter. If a 3 ns TOA correlation window is implemented, at directsunlight (100 k lux) there would still be approximately 12,000correlated arrival events per 30 ms frame time per pixel. Yet theinformation per pixel per frame can be encoded in a single 12 bit output(log₂(200 m/5 cm)), meaning that the information may be extremely sparsein the pixel.

Some in-pixel data reduction or minimization by averaging oftimes-of-arrival for measurement of molecular fluorescence lifetime hasbeen demonstrated. However, such methods may be inadequate for LIDARapplications because in fluorescence lifetime imaging microscopy (FLIM)applications, there is a priori information on the expected TOA offluorescent photons. For example, a laser cycle time of 1 μsec may beutilized with fluorophores with lifetime on the order of 0.5 ns-3.5 ns,which makes it possible to define a very short time window and reject amajority of uncorrelated (also referred to herein as non-correlated)avalanche events. In LIDAR, on the other hand, the echo may arrive anytime within the laser cycle time. Furthermore, in some instances,uncorrelated or non-correlated photon emissions can be reduced orminimized by keeping the system optically isolated, so light onlyoriginates from the pulsed laser, which may not be emitting for a largeduration of the cycle time (e.g., pulse width of 0.5 ns in a cycle timeof 1 μsec). Furthermore, excitation can be kept to short evanescentregions, approximately 50 nm above the waveguide on the top surface ofthe imaging chip. In contrast, in LIDAR applications, ambient lightimpinges the sensor throughout the whole laser cycle, rendering suchspatial isolation of the specific emission difficult. Also, influorescence-lifetime imaging systems, the systems may be designed suchthat the probability of an avalanche per laser cycle is very low,typically below 1%, in order to prevent a statistical error referred toas pile-up. In LIDAR applications, many photons are expected to arriveper laser pulse in certain ambient and target range and reflectivityscenarios. Thus, some problems addressed by embodiments of the presentdisclosure may have been heretofore unaddressed.

FIG. 6 is a block diagram illustrating some in-pixel data reduction orminimization operations in accordance with embodiments of the presentdisclosure. As shown in FIG. 6, upon detection of incident photons (atblock 610), two or more photons having respective times of arrival thatfall within a predetermined correlation time relative to one another arecorrelated (at block 625), for example by a time correlator circuit asdescribed herein. Signals indicative of detection of the correlatedphotons are output for in-pixel averaging (at block 650), which may beimplemented by counter and time integrator circuits as described herein.A point cloud may be generated based on the output of the in-pixelaveraging (at block 670), independent of storing data indicative of thetimes of arrival in a memory (e.g., without storing a TOA histogram asshown in FIG. 5C).

In some embodiments of the disclosure, spectral filtering (e.g. usingone or more optical filters, such as the tunable filters 212, 312) canreduce the ambient light photon counts at the detector (e.g., 110, 210,310) as to allow in-pixel averaging. In some embodiments, this ambientlight rejection is implemented by selecting one or a population of lightemitters (e.g., VCSELs, generally referred to herein as emitters) orarrays thereof (e.g., 115, 215) whose emission spectra are relativelynarrow. In some embodiments, the light emitter or emitter array isattached to a thermally conductive substrate. In some embodiments, thethermally conductive substrate may be used to ensure that the lightemitters are essentially isothermal, and are thermally insulated fromthe surrounding environment. In some embodiments, passive cooling may beused to ensure that the light emitters remain at thermal equilibriumand/or within a temperature range. In some embodiments, the lightemitter and/or the thermally-conductive substrate are actively cooled orheated to maintain them within a desired temperature range. In someembodiments, a temperature sensor (e.g., 213) measures the temperatureof the emitters, the emitter substrate, and/or the ambient temperatureand provides electrical output signals indicative of the temperature(s).In some embodiments, the electrical output signals are converted via alookup table or a mathematical algorithm to a drive signal to a tunablefilter element (e.g., 212, 312). In some embodiments, the emissionwavelength drift of an emitter (e.g., 115, 215) is tracked or measuredusing a spectral measuring device such as, without loss of generality, aspectrometer. The output of the spectral measuring device can beconverted to a drive signal to the tunable filter element (e.g., 212,312). The drive signal may be used to adjust the transmission band ofthe tunable filter element based on the changes in emission wavelengthof the emitter(s), as indicated by the changes in temperature and/orspectral output thereof. In some embodiments an optical filter is placedin front of the emitter or emitter array (e.g., 115, 215) such that theoptical filter selects and outputs a narrower transmission band than thelight output from the emitter(s).

It will be understood that, in some embodiments and without loss ofgenerality, closed-loop control schemes described herein with referenceto the detector portions of a system can be similarly used instead of orin conjunction with the emitter portions of the system. In someembodiments, the temperature control of the emitter may provide a morestable emission band, which is transmitted through a fixed spectralnarrowband filter. In some embodiments the transmission band of atunable filter is tuned so as to follow the emission band of theemitter. In some embodiments, a combination of passive and/or activetemperature controls and active tuning of the transmission band of thetunable filter can reject of ambient light while transmitting thereflected light from the emitter. In some embodiments, the transmissionband of the tunable filter is controlled to follow the same temperaturedependence as the emitters. In some embodiments, the transmission bandof the tunable filter changes with temperature, and an active controlsignal may fine tune the transmission band to match the emission band ofthe emitters.

In some embodiments, a direct TOF imaging system (or “imager”) includesa plurality of pixels. Each of the pixels contains one or two or threeor four or more SPADs (or other photodetectors) with a response timeconfigured to accurately measure the arrival time of photons. In a pixelincluding multiple SPADs, the SPADs may be arranged, for example, in aconcentric (e.g., with a central diode surrounded by one or morering-shaped diodes) or stacked (e.g., with one or more diodes arrangedunder a first diode) arrangement, and where these diodes may eithershare one or more electrical connections or each have their ownelectrical connection. The SPADs are biased such that they are inactive(in a non-operating or non-detecting state, also referred to herein asdeactivated) at least during the firing of the emitter(s) of the LIDARsystem.

Some embodiments provide fast and simultaneous recharging of the SPADarray, such that the SPADs remains biased below breakdown when inactive,and active (in an operating or detecting state, also referred to hereinas activated) and fully charged almost instantaneously, for example,within 0.1 ns, 0.5 ns, 1 ns, 2 ns or 5 ns, subsequent to being inactive.In some embodiments, the recharging of the array is not simultaneous butrather is carried out (e.g., sequentially) for groups of pixels of thedetector array, such as rows, columns or sub-arrays, for example, suchthat each such group can detect reflected optical signals from adifferent set of distances, thus reducing current spikes in thedetector.

In some embodiments, an array of capacitors is provided on the imager(for example, on a same substrate as the SPAD array) so as to allowcharge distribution and fast recharging of the SPAD array. In someembodiments, the array of capacitors is implemented above the substrateof the device. In some embodiments, the array of capacitors isimplemented as an array of Metal-Insulator-Metal (MIM) orMetal-Oxide-Metal (MOM) capacitors which are distributed over areaswhich are not active detection regions (e.g., areas allocated forprocessing circuitry adjacent the SPADs). In some embodiments, largecapacitor banks are implemented above a region of the detector outsidethe SPAD array, while an array of smaller capacitors is interspersedbetween the pixels of the array and/or above interconnect regions. Insome embodiments a second die is bonded to the detector die on a sidethat is not exposed to light, with the second die including an array ofcapacitors for efficient and fast charge distribution.

In some embodiments, a SPAD is connected to the gate of a firsttransistor such that the avalanche output of the SPAD (responsive todetection of an incident photon) switches the state of the firsttransistor. The first transistor is connected to a capacitor. A secondtransistor is connected in series with the first transistor. The gate ofthe second transistor is connected to a global timing circuit. In someembodiments, upon changing its state (responsive to the SPAD beingdischarged), the first transistor is configured to conduct current ontothe capacitor. The global timing circuit changes the state of the secondtransistor upon activation of the SPAD array, so that the secondtransistor is configured to conduct current. In some embodiments, thesecond transistor is turned off by the global timing circuitsynchronously with the pulsed emitter. In some embodiments, current orvoltage integration starts with or is initiated responsive to a globaltiming signal shortly after the emitter pulse and ends upon the earlierof a trigger by an avalanche output of a SPAD or a global end to theactive time window. In some embodiments, current or voltage integrationbegins with or responsive to an avalanche output from a SPAD, and endsjust before the firing of a subsequent emitter pulse. In someembodiments the global timing signals may not be timed with the start ofthe emitter cycles or the end of the emitter cycles, but may be timedbetween the start and the end of the cycle (also referred to herein asstrobing signals). In some embodiments, the timing of the global startand end signals are not identical during all cycles, for example,allowing variable strobing of the range.

In some embodiments, detection of an incident photon and the resultingavalanche output from the SPAD also increments a per-pixel counter. Insome embodiments, the counter is a digital counter. In some embodimentsthe counter is an analog counter which receives a quantum of charge foreach count, with the stored voltage as a measure of the total number ofcounts.

As described above, a SPAD is based on a p-n junction that is biasedbeyond its breakdown region, for example, by or in response to a strobesignal having a desired pulse width. The high reverse bias voltagegenerates a sufficient magnitude of electric field such that a singlecharge carrier introduced into the depletion layer of the device cancause a self-sustaining avalanche via impact ionization. The avalancheis quenched by a quench circuit, either actively or passively, to allowthe SPAD to be “reset” to detect further photons.

In some embodiments, a processing circuit is configured to operateresponsive to photons incident on a detector array by implementingcounter and/or integrator circuits in accordance with embodimentsdescribed herein. The counter and integrator circuits are operable tocount and integrate the individual times of arrivals of the detectedphotons, respectively, in response to the output(s) of one or moredetector circuits that detect incident photons. The processing circuitmay include analog and/or digital implementations of counting circuitsand/or integrator circuits.

In some embodiments, the processing circuit includes a correlatorcircuit (also described herein as a pulse time correlator) that providesoutput signals in response to incident photons that arrive within apredefined correlation window, also referred to herein as a correlationtime. That is, in-pixel correlation as described herein may involvecalculation of the times-of-arrival (TOAs) of signal photons received ina same correlation window defined by the correlator circuit. As such, ifa burst of multiple photons arrive substantially concurrently at a SPADof the array, it has the same effect as a single photon, namely, todischarge that SPAD. Once the SPAD has been discharged by the leadingphoton, it is blind to all the other photons in the burst, while theremaining SPADs in the array may operate likewise responsive to themultiple photons in the burst. The processing circuit may be configuredto calculate an estimated time of arrival of the burst of photons basedon a ratio of the integrated times of arrival (e.g., as provided by timeintegration circuits herein) and the count of the detection (e.g., asprovided by counter circuits herein) of each of the photons byrespective SPADs in the array.

Operation of some embodiments of LIDAR measurement device 900 is shownin FIG. 9. In this example, a distributed charge network 901 deliverscharge to the vicinity of a pixel of a SPAD array, illustrated asincluding two SPADs 910. A global timing generation and reset circuit902 controls the timing of the recharging of the SPADs 910 viarespective recharge circuits 903 coupled to each SPAD 910. In someimplementations, the recharging scheme is passive and, upon an avalancheoutput, the SPAD 910 is immediately and quickly recharged by therespective recharge circuit 903. In some embodiments, the rechargingscheme is active, and the recharge time of the recharge circuit 903 iselectrically controlled in response to respective strobing signalsoutput from the global timing generation and reset circuit 902 (alsoreferred to as active recharge circuitry). An active quench circuit 904,senses the onset of an avalanche and delivers a feedback signal toquickly quench the avalanche.

In some embodiments, the active recharge circuitry 902, 903 biases theSPADs 910 beyond breakdown for respective times correlated with thefiring of an optical signal output from a pulsed light source, such as alaser pulse output from a VCSEL of the LIDAR system. In some embodimentsthe active recharge circuitry 902, 903 biases the SPADs 910 to beactivated for a portion of time (“time gate”), such as a portion of thetime required for a pulse of light to traverse a round trip to thefarthest target and back, and this time gate can be varied so as tostrobe the range of the LIDAR system. In some embodiments, the activerecharge circuitry 902, 903 maintains a SPAD 910 at its recharge statefor duration sufficient to release a relatively large percentage oftrapped charges (for example, 1 ns, 2 ns, 3 ns, 5 ns, 7 ns, 10 ns, 50ns, or 100 ns), and then quickly recharges the SPAD 910.

That is, some embodiments described herein can utilize range strobing(i.e., biasing the SPADs to be activated and deactivated for durationsor windows of time over the laser cycle, at variable delays with respectto the firing of the laser, thus capturing reflected correlated signalphotons corresponding to specific distance sub-ranges at eachwindow/frame) to limit the number of ambient photons acquired in eachlaser cycle. A laser cycle refers to the time between laser pulses. Insome embodiments, the laser cycle time is set as or otherwise based onthe time required for an emitted pulse of light to travel round trip tothe farthest allowed target and back, that is, based on a desireddistance range. To cover targets within a desired distance range ofabout 200 meters, a laser in some embodiments may operate at a frequencyof at most 750 kHz (i.e., emitting a laser pulse about every 1.3microseconds or more).

FIG. 7 is a diagram illustrating relationships between image frames,subframes, laser cycles, and time gates (also referred to herein asstrobe windows) in accordance with embodiments described herein. Asshown in FIG. 7, an example laser cycle may be broken into multiplestrobe windows having respective durations over the time between emittedlaser pulses. For example, at an operating frequency of 750 kHz, a lasercycle may be about 1.3 μs. The laser cycle may be broken into respectivestrobe windows such as, for example, 20 strobe windows. The strobewindows can be mutually exclusive or overlapping in time over the lasercycle, and can be ordered monotonically or not monotonically. In theexample of FIG. 7, the strobe windows may have equal, non-overlappingdurations of 67 ns each, within the 1.3 μs of the laser cycle timebetween laser pulses. An image subframe includes multiple laser pulses,with multiple strobe windows between the laser pulses. For example theremay be about 1000 laser cycles in each sub frame. Each subframe may alsorepresent data collected for a respective strobe window. A strobe windowreadout operation may be performed at the end of each subframe, withmultiple subframes (corresponding to a respective strobe window) makingup each image frame (for example, 20 sub frames in each frame). Thetimings shown in FIG. 7 are by way of example only, and other timingsmay be possible in accordance with embodiments described herein.

FIGS. 8A and 8B illustrate examples of range strobing in accordance withembodiments described herein. In particular, FIG. 8A illustrates nstrobe windows between laser pulses, with each strobe window 0-ndefining a duration of activation for a SPAD at respective delays thatdiffer with respect to the laser pulses, responsive to respectivestrobing signals Strobe#0-Strobe#n. In some embodiments, the strobewindows 0-n are identical in duration, as shown in FIG. 8A. In someembodiments, the strobe windows may be scaled, for example such that“closer” strobe windows (having shorter delays relative to a laser pulsefiring) are wider/of greater duration and “farther” strobe windows(having longer delays relative to the laser pulse firing) arenarrower/of shorter duration, or vice versa, providing non-linear strobewindows, as shown in FIG. 8B.

A time between the pulses of the optical signals (and/or thecorresponding strobe windows) may correspond to a distance range, andthe respective strobe windows may thus correspond to sub-ranges of thedistance range. For example, to image a distance range of 200 meters(m), the respective strobe windows may be defined to cover distancesub-ranges of 1 m-50 m, 50 m-90 m, 90 m-125 m, 125 m-155 m, 155 m-175 m,175 m-190 m, and 190 m-200 m. Such a scheme provides that the strobewindows for acquiring photons reflected from farther targets (which maybe weaker or less reflective) is shorter, thus allowing fewer ambientphotons to arrive over the shorter acquisition window, and therebyachieving a higher signal to background ratio when calculating anaverage time of arrival as compared with a uniform strobe windowduration.

In some embodiments, the number of laser cycles allocated per time gate(e.g., per strobe window readout for each subframe, as shown in FIG. 7,where each subframe indicates data for a respective strobe window) maybe adjusted or varied. In some embodiments, the number of laser cyclesin each frame or subframe covering a specific distance range may beconstant, regardless or independent of the strobe gate width or distancerange. That is, by way of example, for detection at 10 frames per secondusing a 750 kHz laser, each frame may correspond to 75,000 laser cycles(e.g., 75,000 laser pulses for each of 10 time gates (e.g., strobewindow readouts) covering distance sub-ranges of 1 m-50 m, 50 m-80 m, 80m-110 m, 110 m-125 m, 125 m-140 m, 140 m-155 m, 155 m-170 m, 170 m-180m, 180 m-190 m, and 190 m-200 m). In some embodiments, the number oflaser cycles corresponding to each frame or subframe is different fordifferent-range time gates. For example, subframes for farther timegates (with a longer delay from the laser pulse firing, e.g., coveringfarther distances of 190-200 meters) may be allocated or otherwisecorrespond to a greater portion (or a lesser portion) of the number oflaser cycles than subframes for closer time gates (with a shorter delayfrom the laser pulse firing, e.g., covering closer distances of 0-50meters), rather than allocating the number of laser cycles equally pertime gate or subframe. That is, in some embodiments, the number of lasercycles for a distant time gate is larger than the number of laser cyclesfor a closer time gate, or vice versa.

In some embodiments, the relative number of laser cycles in a frame orsubframe for a given time gate scales as the square of the distance. Insome embodiments, background/uncorrelated photon counts are selectivelyrecorded/captured without signal photons (e.g., by suspending firing ofthe laser or by measuring the number of photon counts at a strobe gatecorresponding to a distance range where no target reflects and scalingaccording to the gate width), and the background-plus-signal photoncount is recorded and stored in memory, which can be used to calculatethe signal photon counts (assuming the photon count rate is low enoughto make these parameters independent, or, if they are not independent,correcting for this dependence, e.g., “Pile-Up Correction” schemes). Therelative number of laser cycles per subframe corresponding to a strobegate can be adjusted based on the signal photon count or the backgroundphoton count, or both. For example, the strobing of the detector arraymay be adjusted based on the detected reflectivity of the target (e.g.,based on a feedback indicated by previous detection signals), such morelaser cycles may be allocated to detection of lower-reflectivity targets(e.g., 100,000 of the 750,000 laser cycles may be directed to targets atdistances of 190-200 meters), and fewer laser cycles may be allocated todetection of higher-reflectivity targets (e.g., 50,000 of the 750,000laser cycles may be directed to targets at distances of 0-50 meters), orvice versa. More generally, the number of laser cycles allocated pertime gate/corresponding subframe may be varied so to provide more lasercycles for dimmer (lower-reflectivity) targets, or more laser cycles forbrighter (higher-reflectivity) targets.

In some embodiments, different parts (e.g. SPADs in different regions)of the detector array may strobe different distance rangessimultaneously. In some embodiments, alternate rows of the imager may becharged during different time intervals, which can allow for reducedlaser power while achieving identical SNR, at the cost of lower spatialresolution. For instance, in one example, 120 rows scan at 10 strobewindows, with all rows scanning the same strobe window at a givensub-frame. For a global frame rate of 30 frames per second, the readoutmay be at 300 frames per second, so enough energy must be delivered tothe target each 1/300=3 msec (millisecond) sub-frame. In a furtherexample, alternate rows image alternating strobe windows so that theeffective row number is 120/2=60. For a global frame rate of 30 framesper second, the same energy should be delivered to the target during aperiod of 2/300=6 msec, so the average and peak power can be halvedversus the previous example described above.

In some embodiments, the LIDAR measurement device 900 is shown in FIG. 9may further include a processor that is configured to receive signalsfrom the detector array 910 in response to detection of signal photonsor other avalanches such as thermally-generated avalanches, determine abrightness of a respective target based on the received signals, anddynamically adjust the number of laser cycles, correlation window forthe detectors, strobe window of the detectors, and/or backgroundsubtraction/correction as described herein based on the determinedbrightness of the target.

The LIDAR measurement device 900 FIG. 9 further includes a pulse timecorrelator 925 (also referred to herein as a correlator circuit), a timeprocessing circuit 950 including an event counter 950 a (also referredto herein as a counter circuit) and a time integrator 950 b (alsoreferred to herein as a time integrator circuit), a saturation controlcircuit 955, an array sequencer and readout circuit 960, and a processorunit 970 (which may be provided on-chip or off-chip). In someembodiments, the array sequencer and readout circuit 960 may operate incoordination with the emitter array (and/or related control circuit) ofthe LIDAR system. The correlator circuit 925, counter circuit 950 a, andtime integrator circuit 950 b are described in greater detail below.

Some embodiments of the present disclosure provide various architectureswhereby the counter circuit 950 a is implemented as an analog eventcounter and/or the time integrator circuit 950 b is implemented as ananalog time integrator, in combination with correlator circuits 925and/or saturation control circuits 955 as described herein. For example,as shown in FIG. 10, a SPAD 1010 is connected to a passive quench andrecharge transistor 1003 with static DC bias voltage “VO” controllingthe recharge or “dead” time of the SPAD 1010. This is connected to ananalog counting Charge Transfer Amplifier (CTA) circuit 1050 a via a twotransistor global shutter time gate 1047. The CTA circuit 1050 a isreset by pulling the main capacitor “C” to the high reset voltage VRT.The CTA circuit 1050 a operates responsive to the input gate voltage (inthis case the anode voltage of the SPAD 1010) increasing above thethreshold voltage of the input source follower. Charge flows from themain capacitor “C” to the parasitic capacitor “CP” and the voltage riseson the parasitic node. The rising voltage pushes the source followerinto the cut-off region and the charge flow halts, causing a discretecharge packet to be transferred from the main capacitor for each inputpulse. The SPAD 1010 begins recharging and the lower transistor 1051 inthe CTA circuit 1050 a discharges the parasitic capacitance which isachieved with a static bias voltage “VDC” applied keeping thistransistor, below threshold, in weak inversion.

FIGS. 11A and 11B are provided by way of example to illustrate operationof time processing circuits 1150 including analog counter circuits 1150a and analog time integrator circuits 1150 b in accordance withembodiments described herein. Timing of an associated emitter array(Laser pulses) is also illustrated. The counter circuit 1150 a operatesby injecting a fixed amount of charge onto a capacitor Ca. Thus, thevoltage on the capacitor Ca (V_event_counter) is the number of recordedavalanches times the charge quantum divided by the capacitance of Ca.The analog time integrator circuit 1150 b operates by starting to injectcurrent to a different capacitor Cb at the onset of an avalanche, andstopping at a globally-controlled time (e.g., based on a referencetiming signal). Thus, the voltage on the capacitor Cb (V_time_accum)equals the current flowing (I) times the total integrated time dividedby the capacitance of Cb. The pixels are read out (Read_pixel_caps) atthe end of a subframe, providing a count value (Number_of_events) and asum of the arrival times (sum(arrival_times) for the subframe, fromwhich an average time of arrival of detected photons can be estimated.

In some embodiments of the present disclosure, each pixel includes 2SPADs with their guard-rings and optical and electrical isolation. Theleading edges of the SPAD's output in response to an avalanche aretime-correlated in a compact, in-pixel pulse-time correlator (PTC). Ifthe leading edges arrive within a pre-set or tunable “correlation time”,then the latter of the leading edges will be transmitted, withrelatively low jitter, to the time processing circuitry, comprising theevent counter and the time integrator. If there are no correlatedavalanches, no signal will reach the time processing circuitry. In someembodiments, only one (e.g., the earlier one) of the edges will beprocessed. In some embodiments, both edges will be processed and twocorresponding sets of event counters and time integrators areimplemented in the pixel. In some embodiments, the PTC only outputs adigital signal of one polarity if two detected avalanches aresufficiently close in time (e.g., within the predetermined correlationtime) and otherwise outputs the other polarity, and this correlationsignal serves as a control for a switch which allows timing and eventmeasurements for correlated events, and otherwise does not allow such ameasurement to occur.

In some embodiments, digital circuits may replace the time integratingcapacitor Cb and/or the event counter capacitor Ca. In some embodiments,the output of the correlator (e.g., 925) is fed to a time-to-digitalconverter (TDC) with a dynamic range corresponding to the ratio of thetime duration of the strobe gate (e.g., 66 nsec for a 10 meter window)and the required or desired temporal resolution per measurements (e.g.,2 nsec)—in this example only 5 bits are required—which may besignificantly fewer than in other LIDAR pixels which typically require12-14 bits and thus occupy more space in the pixel. The output of theTDC can be stored in memory in the pixel. A digital accumulator (e.g.950 b) adds the arrival times. Similarly, a digital counter (e.g., 950a) increments after each correlation event output from the correlator(e.g., 925). The values of the digital time accumulator and the digitalevent counter are likewise read out at the end of a subframe, from whichan average time of arrival of detected photons can be estimated.

In some embodiments, the events are recorded in memory in telegraph codewith a number of bits equal to the ratio between the strobe gateduration and the required or desired temporal resolution. For example,if the strobe gate is 66 nsec and the required or desired measurementresolution is 2 nsec, 33 memory bins are used, each with a number ofmemory cells. The number of memory cells may be determined by the totalexpected count per time bin. For each arrival, the memory value at theappropriate bin is incremented, thus generating a real-time histogram.In some embodiments, the inputs to this memory array are not direct SPADoutputs but rather correlated events, thus resulting in a much “cleaner”and smaller histogram, with significantly fewer uncorrelated events. Itwill be understood that the area occupied by the memory cells can be alimiting factor in implementation, and therefore the significantreduction of events to be processed (by using the correlator and/or thestrobing gates in accordance with embodiments described herein) may bedesirable.

In some embodiments, the two SPADs or microcells in a pixel of adetector array are defined by two or more diodes, each enclosed in aguard ring. In some embodiments, as shown in the plan andcross-sectional views of FIGS. 12A and 12B, respectively, a pixel 1200includes two diodes D1 and D2 arranged concentrically, e.g., such that afirst, central diode D1 is elliptical or polygonal, and a second diodeD2 defines an elliptical or polygonal ring surrounding the central diodeD1, thus reducing the total pixel area occupied by the pair of diodesD1, D2. In some embodiments, isolation structures, such as trench orimplant structures, may be formed to better isolate the two diodes D1,D2. More generally, the second diode D2 may be arranged around aperimeter or periphery of the first diode D1. The diodes D1, D2 aresufficiently electrically and optically isolated from each other andeach have a readout node as described herein. In some embodiments, morethan two concentric diodes may be formed in the pixel 1200. For example,a third diode may likewise define an elliptical or polygonal ringsurrounding the diode D2, a fourth diode may define an elliptical orpolygonal ring surrounding the third diode, and so forth.

In some embodiments, the two or more correlating diodes in a pixel maybe vertically stacked. For example, the two diodes D1, D2 may beimplemented using different junctions in the pixel. As an example andwithout loss of generality, a pixel 1300 may include one diode D1defined by the source-drain diffusion to N-well junction (Junction 1)and the other diode D2 defined by the N-well to Deep N-well junction(Junction 2), as shown in the cross-sectional view of FIG. 13. However,it will be understood that embodiments of the present disclosure are notlimited to the illustrated junction structures, and other junctionstructures may be used to provide sufficient electrical isolation suchthat an avalanche in one diode has a low probability of inducing anavalanche in the second diode. In some embodiments, each diode has itsown readout circuit. In some embodiments, the pixel 1200, 1300 or otherstructure has a single readout where one diode's avalanches are read outas one voltage polarity and the other's avalanches are read out as asecond polarity, both feeding into the readout circuit as describedherein. A reduction in pixel area and thus significant pixel area savingcan thereby be achieved. In some embodiments, the two or more diodes ofa pixel are stacked via wafer-to-wafer bonding with appropriateelectrical interconnection. Appropriate computational correction factorscan be made by processing circuitry as described herein to account forthe different detection probabilities of the two diodes.

It will be understood that signal and gate polarities described hereinare provided by way of example only and may be changed without loss offunctionality in accordance with embodiments of the present disclosure.The time processing circuitry (e.g., 950) includes a counter (e.g., 950a) and a time integrator (e.g., 950 b). Both the counter and the timeintegrator may be reset once per frame. Both the counter and the timeintegrator may be disabled by a saturation control circuit (e.g., 955).

An example saturation control circuit 1455 in accordance with thepresent disclosure is illustrated in FIG. 14, the operation of whichwill now be described. In FIG. 14, DC voltages Time saturation and Countsaturation are provided to the pixel. Comparators X1 and X2 monitor thevoltage outputs TI_out and EC_out of the time integrator 1450 b andevent counter 1450 a, respectively. Once either voltage (TI_out orEC_out) reaches a threshold set by the external voltages (Timesaturation and Count saturation), the output of the comparator circuitX1, X2, (saturated) switches to 1, and freezes the values of the timeintegrator 1450 b and event counter 1450 a. The time integrator 1450 band event counter 1450 a may be reset by respective reset signals at thebeginning of a frame. An external End_frame signal is input to an ORgate 02. If either the time processing circuitry 1450 reaches saturationor the End_frame flag is 1, a data-ready bit becomes 1. In someembodiments the event counter 1450 a is an analog counter. In someembodiments the event counter 1450 a is a digital counter.

In some embodiments, the PTC (e.g., 925) is configured to output onebinary data as a correlation signal when its two inputs, which are thedirect, inverted, or buffered outputs of the two SPADs (e.g., 910),arrive within a predefined or predetermined duration of time (alsoreferred to herein as a “correlation time” or “correlation window”), andotherwise outputs another binary output or a tri-state output. In someembodiments, the PTC may provide the direct, inverted, or bufferedoutput of a first SPAD and a second SPAD of a detector array to theclock input and the data input of an edge triggered D Flip Flop.

An example pulse time correlator (PTC) circuit 1525 in accordance withthe present disclosure is shown in FIG. 15, the operation of which willnow be described. As throughout the present disclosure, the polarity ofgates may be reversed in particular embodiments. As shown in FIG. 15,The PTC 1525 provides the direct or buffered voltage of the switchingnode of each of the two or more SPADs (SPAD1, SPAD2) of a detector array(e.g., 910) to respective AND gates (AND1, AND2), directly and via delayelements (BUF1, BUF2). In some embodiments, the delay through the delayelements BUF1 and BUF2 is equal to the desired correlation time. In someembodiments, the delay elements BUF1 and/or BUF2 may be implemented asan odd number of inverters connected in series. In some embodiments, thedelay elements BUF1 and/or BUF2 may be implemented as even number ofinverters connected in series. In some embodiments, the delay providedby elements BUF1 and/or BUF2 can be controlled via external circuitry.In some embodiments, the delay provided by elements BUF1 and/or BUF2 canbe controlled to maintain desired levels across multiple manufacturingprocess conditions, supply voltage variations, and/or temperatures(PVT). The outputs (A,B) from the AND gates (AND1, AND2) are fed to anOR gate (OR) and an AND gate (AND), which generate the Data and CLKinputs, respectively, for an edge-triggered D Flip Flop 1520,respectively. A Reset signal (Reset) is used to reset the output of theflip flop 1520. The flip-flop 1520 is configured such that its setuptime is approximately equal to or less than the maximal correlation timecorresponding to a correlated detection. In some embodiments, thecorrelation time is shorter than the pulse duration of an emitter pulse.In some embodiments, the correlation time is longer than the pulseduration of an emitter pulse. The PTC circuit 1525 is configured suchthat it adds a reduced or minimal possible jitter to the data when datais propagated to its output.

The timing diagrams including the waveforms shown in FIGS. 16A-16Cillustrate example operations of the PTC circuit 1525 shown in FIG. 15.In FIGS. 16A-16C, the signal “Out” refers to the output of the PTCcircuit 1525, also referred to herein as a correlation signal. At theend of the measurement window, the state of the flip flop 1520 is resetby the signal Reset.

The timing diagrams including the waveforms shown in FIG. 16A illustratethe operation of the PTC circuit 1525 when the detection of avalanchesin two SPADs (SPAD1 and SPAD 2), output as detection signals SPAD_1 andSPAD_2, occur with a lag time relative to one another that is longerthan the predetermined correlation time. The detection signals SPAD_1,SPAD_2 indicate respective times of arrival of photons at thecorresponding detector elements (SPAD1, SPAD 2). The buffer (BUF1, BUF2)and AND gates (AND1, AND2) convert the leading edge of each detectionsignal SPAD_1, SPAD_2 from a respective avalanche into pulse signals A,B having pulse widths or durations corresponding to the correlationtime. The signal A+B output from the OR gate (OR) as the data signal tothe flip flop 1520 indicates that the pulse signals A, B, do notoverlap. The CLK signal A×B samples a digital 0 and is provided to Dflip flop 1520, such that the correlation signal Out generated by thePTC circuit 1525 is 0, indicating that the respective times of arrivalof the photons detected at SPAD1 and SPAD 2 are not correlated.

The timing diagrams including the waveforms shown in FIG. 16B illustratethe operation of the PTC circuit 1525 when the detection of avalanchesin the two SPADs (SPAD1, SPAD 2), output as detection signals SPAD_1 andSPAD_2, occur just within the predetermined correlation time. In thiscase, the signal A+B output from the OR gate (OR) as the data signal tothe flip flop 1520 indicates that the pulse signals A, B, overlap, andthe rising edge of the CLK signal A×B samples a digital 1 such that acorrelation signal Out generated by the PTC circuit 1525 is 1,indicating that the respective times of arrival of the photons detectedat SPAD1 and SPAD 2 are correlated.

The timing diagrams including the waveforms shown in FIG. 16C illustratea scenario when the detection of avalanches in two SPADs (SPAD1 and SPAD2), output as detection signals SPAD_1 and SPAD_2, occur substantiallysimultaneously. In this case, the signal A+B output from the OR gate(OR) as the data signal to the flip flop 1520 also indicates that thepulse signals A, B, overlap, and the rising edge of the CLK signal A×Bis provided to the flip flop 1520 with the data signal A+B. Because thesetup time of the flip flop 1520 is set to zero, the data A+B is stillsampled correctly and a correlation signal Out generated by the PTCcircuit 1525 is 1, indicating that the respective times of arrival ofthe photons detected at SPAD1 and SPAD 2 are correlated.

Specific implementations of correlator circuits in accordance withembodiments of the present disclosure have been provided as an example,but are not so limited. As such, other correlator circuits which providea binary signal output of one type when avalanches occur within thecorrelation time, and another binary signal output when no two pulsesoccur within the correlation time, can be implemented in accordance withembodiments of the present disclosure.

In some embodiments, the number of events and the sum of times arestored as voltages on capacitors (e.g., Ca and Cb, respectively).

In some embodiments, only one TOF measurement is made per laser cycle.In some embodiments, multiple TOF measurements are made in one lasercycle.

In some embodiments, only one event counter and time integrator pair(e.g., 950 a and 950 b) is included in a pixel. In some embodiments,more than one pair of event counters and time integrators is included ina pixel such that if one pair has already been triggered to record atime and event, the next pair is used.

In some embodiments, a rolling shutter readout scheme is used to readout the voltages from the pixels one row at a time. In some embodiments,a global shutter scheme is used to read out all voltage of the detectorarray at once.

In some embodiments, a Region of Interest (ROI) is defined whereby onlya subset of the detector array is read out.

In some embodiments, a circuit is included in the pixel to calculate theratio of the integrated time and the number of events to derive theaverage time of arrival. For example, a Gilbert Multiplier circuit maybe integrated in the pixel.

In some embodiments the read-out voltages are digitized using an analogto digital converter (ADC). In some embodiments, the ADC is on-chip. Insome embodiments the ADC is off-chip. In some embodiments the read-outis in a bonded chip (e.g., a readout integrated circuit (ROIC)).

In some embodiments the imaging chip which includes the SPAD array (andin some embodiments, a CIS array) is frontside illuminated. In someembodiments the imaging chip which includes the SPAD array is backsideilluminated.

In some embodiments, an on-chip or off-chip processing unit (e.g., amicroprocessor) calculates the ratios of integrated times (voltages) tonumber of events (voltages) per pixel. In some embodiments a processingunit converts all ratios to ranges and azimuth-height coordinates andstores and/or displays the results as a 3D point cloud.

Other advantages of embodiments of the present disclosure may includeimproved dynamic range. Dynamic range may be an obstacle for all othermodalities, including PMDs. Some example calculations are providedbelow; however, it will be understood that these examples arenon-limiting and provided for purposes of illustration only.

In the following examples with reference to LIDAR systems, it is notedthat illumination photon flux (photons/area/time) typically falls off asa square of the distance (in some cases, e.g., when a non-divergent beamis used, the illumination photon flux remains approximately constant,but this is not the typical configuration for long-range LIDAR systems).Therefore, a target 200 m away may be illuminated with(5/200){circumflex over ( )}2=6.25e-4 as much power as a target 5 m away(with the assumption that 5 m is a minimum desired detection range). Inthis example, the closer target is a specular reflector (100%reflectivity) and the farther target has 10% reflectivity and is aLambertian reflector. The reflected power received by the detector alsogoes as the square of the distance range, so the echo from the fartarget may again be 6.25e-4 of the echo from the close target.Therefore, some integrating detectors, even before considering ambientlight, may deal with a dynamic range of 1:25.6 million. However, in someimage sensors, typical full well capacity may be about 5,000 to 100,000(a sensor meeting these specs may need a full well capacity of 25.6million electrons, if it could read out with a read noise of less thanone electron, which may be not be possible). When the effects ofsunlight are included, which may add a background of about 500 photonsper cycle, this problem may become significantly worse.

In contrast, embodiments of the present disclosure using single photondetectors can paradoxically address dynamic range problems(paradoxically because single photon detectors can handle high fluxesbetter than pin diodes, APDs, PMD, etc.). If a large burst of photonsarrives at a SPAD, it has the same effect as a single photon, namely, todischarge the SPAD. Once the SPAD has been discharged by the leadingphoton, it is blind to all the other photons in the burst. Manyconcurrent photons may increase the probability for a correlated-pairdetection, but otherwise operation of the SPAD may be unaffected.Embodiments of the present disclosure may thus provide furtheradvantages, particularly where ambient photons are rejected andsensitivity is tuned for detection of farther targets.

Further improvements to the estimation of the center of mass by thepixels described above in the presence of uncorrelated photons aredescribed herein. In the equations below, t_(calc) is the time measuredby the pixel, t_(sigwidth)/2 is half of the temporal spread of theavalanches correlated with the signal, s(t) is the average count rate ofcorrelated events within the t_(sigwidth), S is the total number ofcorrelated events per subframe, b(t) is the average rate of uncorrelatedevents passing through the correlator, and B is the total number ofuncorrelated events passing the correlator per subframe. b(t) and B canbe calculated, as described herein, during a strobe gate not containinga signal echo, or during an interval when the emitter is not firing.t_(s) is the real or actual time of flight to the target (which is agoal of the estimation).

${\left. {t_{calc} = {\frac{{\int_{t = {t_{s} - t_{{sigwidth}/2}}}^{t_{s} + t_{{sigwidth}/2}}{{{ts}(t)}{dt}}} + {\int_{t = 0}^{t_{strobe}}{{{tb}(t)}{dt}}}}{S + B} = \frac{{st}^{2}}{2\left( {S + B} \right)}}} \right\rbrack_{t_{s} - t_{{sigwidth}/2}}^{t_{s} + t_{{sigwidth}/2}} + \frac{{bt}_{strobe}^{2}}{2\left( {S + B} \right)}} = {{{\frac{s}{2\left( {S + B} \right)} \times \left\lbrack {\left( {t_{s} + t_{{sigwidth}/2}} \right)^{2} - \left( {t_{s} - t_{{sigwidth}/2}} \right)^{2}} \right\rbrack} + \frac{{bt}_{strobe}^{2}}{2\left( {S + B} \right)}} = \frac{{4{st}_{s}t_{{sigwidth}/2}} + {bt}_{strobe}^{2}}{2\left( {S + B} \right)}}$$\mspace{20mu} {t_{s} = \frac{{2\left( {s + b} \right)t_{calc}} - {bt}_{strobe}^{2}}{4{st}_{{sigwidth}/2}}}$$t_{s} = {\frac{{2{t_{calc}\left( {S + B} \right)}} - {bt}_{strobe}^{2}}{4{st}_{{sigwidth}/2}} = {\frac{{2{t_{calc}\left( {S + B} \right)}} - {bt}_{strobe}^{2}}{2{st}_{FullWidth}} = {\frac{t_{calc}\left( {S + B} \right)}{{st}_{FullWidth}} - \frac{{bt}_{strobe}^{2}}{2{st}_{FullWidth}}}}}$  t_(FullWidth) = 2t_(sigwidth/2)$\mspace{20mu} {{b = \frac{B}{t_{strobe}}};{s = \frac{S}{t_{FullWidth}}}}$$\mspace{20mu} {t_{s} = {\frac{t_{calc}\left( {S + B} \right)}{S} - \frac{{Bt}_{strobe}}{2S}}}$

-   -   Where

$t_{FullWidth} \sim {\sqrt{t_{{vcsel},{pw}}^{2} + t_{{spad},{jitter}}^{2}} + {\frac{1}{2}{t_{correl}.}}}$

The square root is the temporal spread.

In some embodiments, CMOS Image Sensor (CIS) pixels, such as ActivePixel Sensor (APS) pixels or Passive Pixel Sensor (PPS), may beintegrated within a SPAD-based detector array, and the respectivecontroller array, which may be used for either rolling-shutter readoutor global shutter read-out, may be provided either on the same die or ona separate readout integrated circuit (ROIC). A detector sub-array 1710according to some embodiments of the present disclosure is shown in FIG.17. In this example, SPAD pixels 1702 include SPAD active area 1701,with surrounding guard ring and circuitry. Also included in the detectorarray 1710 are CMOS Image Sensor (CIS) pixels 1703. Thus, two outputscan be generated from the detector array 1710—one being the 3D pointcloud (based on the detection signals output from the SPADs 1702), andthe other a gray scale undersampled image of the same field of view(based on the outputs of the CIS pixels 1703).

In some embodiments, the output of the CIS pixels 1703 and the SPADpixels 1702 can be combined or “fused” by a processing unit to form anintensity-encoded 3D point cloud. In some embodiments, a separate ordedicated image sensor device can generate a complete image, eithermonochrome or color. In some embodiments, the undersampled image fromthe CIS pixels 1703 can be fused with the image from the separate ordedicated image sensor device, with the undersampled features serving asfiducials or registration points for the image from the separate ordedicated image sensor device. In some embodiments, this fused image canalso be fused with the 3D point cloud generated by the SPAD pixels 1702.

In some embodiments, a SPAD imager may be packaged with an image sensorthat is sensitive to infrared (IR) photons, for example, to photons withwavelengths longer than 1200 nm. In some embodiments, the infrared imagesensor can be bonded to a silicon-based SPAD device such that visiblephotons are absorbed by the silicon-based SPAD device but infraredphotons (which cannot be absorbed by the silicon-based detector) willpass through to the IR sensor. It will be understood that, in such anembodiment, the optical system can have a sufficiently low numericalaperture and/or depth of focus so as to allow both focal plane arrays toconcurrently be in focus, and/or that a tunable focus can be integratedso as to allow either detector (SPAD or IR) to be in the focal plane ofthe system. It will also be understood that metallization in the SPADdevice can be designed such that it allows IR photons to reach thebonded IR sensor. It will also be understood that a filter at the front(or input) of the detection module or array can be configured such that,in the visible range, the filter will transmit a relatively narrowspectral band, e.g., having a transmission band of about 1 nm or 5 nm or20 nm around 930 nm or 940 nm wavelength of light, and the filter canconcurrently function as a high pass filter in with respect to IRwavelengths of light, transmitting as wide a band as desired above 1200nm, as shown for example in the graph of FIG. 18A.

FIG. 18B illustrates an example of an integrated visible TOF-IR imagesensor device 1800 in accordance with embodiments of the presentdisclosure. A SPAD die 5 (for example, a silicon die) is bonded, eitherwafer-level or die-level, with an IR detector die or wafer 6 (forexample, an InGaAs die or wafer). The SPAD die 5 includes a top sectioncontaining metallization lines 2, wire bonding pads 1, and aninter-metal dielectric, with a lower or bottom part 4 containing thephotodiodes 3. The IR die 6 may be backgrinded and backside illuminated(BSI) so that its photodiodes 7 are on the top-facing side of the die 6.In some embodiments, the IR die 6 is not backgrinded or is frontsideilluminated. In a BSI IR sensor, metallization lines 10 and microbumpsare used to electrically interconnect the IR array 6 to a substrate 11.In some embodiments, wirebonds 12 interconnect the SPAD array 5 to thesame substrate. In some embodiments, thermal control may be provided toreduce the dark current from the IR detectors 7 to a sufficiently lowlevel while maintaining the afterpulsing rate from the SPAD detectors 3sufficiently low.

Some further embodiments of the present disclosure are directed to delaymeasurements on SPAD pixels based on area calculation with a background(BG) reference. Such embodiments may be used in combination with theanalog or digital circuits used to compute time of flight inside eachpixel (e.g., counter and time integrator circuits) as described above,as well as with features described above including (1) tunable narrowbandpass filters; (2) detector strobing or gating, particularly gatewidths that vary with time of flight; and/or (3) coincidence counting(e.g., using correlator circuits to detect photons arriving within apredetermined correlation time relative to one another) to bias countingtoward desired signal photons (e.g., from a desired light source) overbackground photons (e.g., from ambient light sources).

In particular embodiments, a second “reference” channel, which isconfigured to permit background photons (also referred to herein as BGor bg photons) to enter, may be used. The reference channel may beimplemented by one or more single photon detectors that are configuredto detect incident photons that are outside of the wavelength range ofthe optical signals emitted by a pulsed light source for a LIDAR system,such as VCSEL arrays describe herein. This reference channel isconfigured to estimate statistics related to only the backgroundphotons, and thereby allow for correction of the estimates of the timeof flight on the “main” channel (signal+background). As backgroundincreases, the TOF estimate may be “pulled” toward the center of thegate; thus, the reference channel according to further embodimentsdescribed herein provides an analog output which can be used tocounteract the pull toward the center.

In some embodiments, the reference channel may be implemented by opticalfilters that are arranged to provide the input light to the respectivesingle photon detectors of a detector array. Some of the optical filtershave transmission bands that permit light of the desired signalwavelength and background wavelengths, while others have transmissionbands that permit light of the background wavelengths only. That is, thereference channel includes an optical bandpass filter that is not on(i.e., does not correspond to or is otherwise configured to preventtransmission of) the wavelengths of the desired signal photons. Othermethods to deprive the reference channel of desired signal photons mayalso be implemented. In some embodiments, microtransfer printing may beused to apply the respective optical filters to respective single photondetectors of the detector array.

As described in some examples herein, SPADs may be an example ofdetectors that can be used for detecting extremely low levels of signal.However, a SPAD may register an event for every photon that it detects,including background photons. This means that even if the signal couldbe recovered, the memory and processing requirements (fordigitizing/recording/analyzing all of the photon events) may likely betoo great to include in a single pixel, if the analysis strategy relieson analysis using all of the photon events.

As discussed in detail above, embodiments of the present disclosure aredirected to light ranging detection and analysis that provides areduction in the data load of SPAD output at the point of detection,without losing essential information (e.g., information that indicatesthe time of flight of the desired signal photons). Some embodiments maythus “integrate” all events “on the fly” into a small number of “figuresof merit” from which the desired time of flight can be estimated.

For example, some embodiments may calculate the average delay (e.g.,first moment of histogram of time of flight from all SPAD avalancheevents). FIG. 19 illustrates an example analog time processing circuit1950 that is configured to compute the mean time of flight value byindependently computing two quantities: (1) the sum of the TOF (or TOA)of all events (e.g., integrated on a capacitor Cb); and (2) counter ofthe number of events (e.g., stored on capacitor Ca). This circuit 1950may be suitable for estimating TOF; however, background photons (andother sources) may result in detection events which are not from lasersignal echos, providing noise in the measurements. As such, withoutadditional information, the values generated by the circuit 1950 may, insome instances, mix desired signal and background photons into the samevalue.

Some further embodiments may thus generate additional information thatcan improve extraction of values that correspond to the desired signalphotons (e.g., laser signal echos). For example, in some embodiments, anindependent estimate of the average background intensity (and delayvalue) may be generated by implementing a reference detection channel #2using similar devices as the primary channel #1. The reference detectionchannel #2 may operate simultaneously/in parallel with channel #1, ormay operate sequentially/before or after channel #1. Channel #2 differsfrom channel #1 in that it is configured so as not to register or detectdesired signal photons. By using the time processing circuit 1950 shownin FIG. 19 (or similar circuits) on both the primary and referencechannels, but tuned for different wavelength ranges (e.g., usingrespective optical filters), further embodiments may provide detectionanalysis that allows the desired signal photons to be distinguished fromthe background photons, based on relationships between the values outputfrom the primary and reference channels.

Some further embodiments described herein may thus allow analogcalculation of peak delay by average of integrator by making use of areference BG channel. To this end, a monte carlo analysis may be used totreat quantities as random variables drawn from a Gaussian distributionwhose standard deviation (sigma) is equal to the square root of theexpectation value (mean). The results can be expressed in terms of themost likely estimate of time of flight as well as the variance of thatestimate. Each (estimate and variance) are plotted as a function ofdesired signal (s) and background (b) which are varied independently,where s=total number of desired signal photons during a time gate andb=total number of background photons during a time gate.

FIGS. 20A and 20B illustrate examples for distinguishing photons fromsignal and background across the time gate (Tg; also referred to hereinas Tgate). To compute the average delay of the desired signal photonsonly <t>s, analog circuits as described herein may be configured tomeasure the average delay <t> and total photon counts, A, from a givenphotodetector. In particular, two channels may be used: one with adetector that is configured to collect light for a band/wavelength rangethat contains both desired signal and background photons, and the otherfor a band/wavelength range that contains just background photons. Theequations below show how the desired quantity, average delay of signalonly, can be derived from these measurements:

${\langle t\rangle}_{b + s} = {\frac{{{\langle t\rangle}_{b}A_{b}} + {{\langle t\rangle}_{s}A_{s}}}{A_{b} + A_{s}} = \frac{{\frac{1}{2}T_{g}A_{b}} + {{\langle t\rangle}_{s}A_{s}}}{A_{b} + A_{s}}}$$M_{1} = \frac{{\frac{1}{2}T_{g}M_{3}} + {{\langle t\rangle}_{s}\left( {M_{2} - M_{3}} \right)}}{M_{2}}$${\langle t\rangle}_{s} = \frac{{M_{1}M_{2}} - {\frac{1}{2}T_{g}M_{3}}}{M_{2} - M_{3}}$⟨t⟩_(b + s) = measurement1 = M 1A_(b) + A_(s) = measurement 2 = M 2 A_(b) = measurement 3 = M 3⟨t⟩_(b) = measurement 4 = M 4

FIG. 21 is a graph illustrating the addition of noise to calculations,based on a given number of signal and background photons on channel #1,where S=the quantity of desired signal photons and B=the quantity ofbackground photons (or dark counts). A random number of backgroundphoton counts may be drawn on (reference) channel #2: b_ref=b+X, notingthat b_ref may not necessarily be the same value as b.

Photons may be distributed across Tgate according to expecteddistributions, and values for M1, M2, M3, M4, M5, M6 are computed, whereM5=M3 using b_ref, and M6=M4 using b_ref. Estimates are computed basedon 3 different equations:

${\langle t\rangle}_{s} = \frac{{M_{1}M_{2}} - {M_{6}M_{5}}}{M_{2} - M_{5}}$${\langle t\rangle}_{s} = \frac{{M_{1}M_{2}} - {\frac{1}{2}T_{g}M_{3}}}{M_{2} - M_{3}}$${\langle t\rangle}_{s} = \frac{{M_{1}M_{2}} - {M_{4}M_{3}}}{M_{2} - M_{3}}$

The above operations may be repeated for N-iterations to generateexpected estimates and uncertainty of signal delay, ts. The accuracy ofthe calculation of TOF may thus increase as background noise is reducedin accordance with embodiments described herein. Accuracy may be furtherincreased by increasing pulse power to increase photons per pulse, andSNR may be increased by integrating more pulses.

In embodiments including a single-photon detector-based referencechannel that is configured to detect uncorrelated background (BG)photons for correction of the estimates of the time of flight on the“main” channel (which is configured to detect an aggregate of correlatedsignal photons+BG photons), the system may operate in three regimes. Ina first regime, background counts are negligible in the sense that theyhave a sufficiently negligible effect on the center of mass calculationin the pixel. In a second regime, background counts are sufficientlyhigh that the background collection operations described herein can beapplied and, if the signal level is sufficiently high, the operationscan estimate the correct range with a sufficiently low error. In a thirdregime, the background level is high enough to affect the measurementerror adversely, but is too low to assume a uniform background countacross the cycle. In some embodiments described herein, it may bedesirable to operate in the first or third regimes and to avoid thesecond regime.

One way to move the system from the third regime to the first regime, ifthe signal level is sufficiently high, is to permit detection of moreambient light. This may be counterintuitive because, for a conventionalimage sensor, this will result in increased background noise and thus ina lower signal-to-noise ratio. However, in pixel implementationsemploying background correction according to some embodiments describedherein, the detection of background photons with higher uniformityreduces the error of the background-correction operations.

Some embodiments described herein may thus include processing circuits(e.g., correlator circuit 925) that are configured to adjust (e.g.,increase or decrease) a correlation time window based on detected levelsof background (uncorrelated) photons, signal (correlated) photons,and/or a ratio of background-to-signal photons. In some embodiments, anincrease in uncorrelated photons can be achieved by increasing thebandwidth of the spectral filter (e.g., tunable filter 212) to allowmore non-emitter signal (e.g., ambient light) in. In some embodiments,an increase in the uncorrelated photon counts per frame can be achievedby broadening the correlation window of the in-pixel correlator. Forexample, the correlation window can be altered by electronicallycontrolling the delay in the buffers BUF1 and/or BUF2 in the Pulse-TimeCorrelator (PTC) circuit 1525 of FIG. 15, thus changing the width of thepulse signal A and/or B. When the correlation time is longer, moreuncorrelated counts can be passed to the Event Counter (e.g., 1450 a)and Time Integrator (e.g., 1450 b), transitioning the system to thefirst regime. In some embodiments, the uncorrelated photon count levelis monitored or detected in a passive frame (i.e., where the emittersare not firing or emitting) or in a strobe gate where there is notarget/reflection, and the correlation window is adjusted accordingly inresponse to the uncorrelated photon count level. In some embodiments,the correlation window is controlled globally for the single photondetectors of the detector array. In some embodiments, the strobe windowis controlled by region of the detector array, that is, such thatdifferent correlation times are applied to detection signals output fromdetectors in a first region of the detector array and detection signalsoutput from detectors in a second, different region of the detectorarray. In some embodiments, the strobe window is controlled for one ormore pixels of the detector array.

That is, some embodiments may include determining a background(uncorrelated photon) detection level relative to a threshold (which maybe based on a predetermined or desired background detection level, apredetermined or desired signal level, and/or a ratio therebetween), andthen adjusting (e.g., increasing or decreasing) the correlation timethat is applied to detection signals from one or more of the singlephoton detectors when the determined background detection level is belowthe threshold. This allows for detection of background photons withhigher uniformity, which can increase accuracy of backgroundsubtraction/correction operations described herein.

In some embodiments, the correlator circuit (e.g., 1525) may be bypassedand the signal from either one or both SPADs may be driven directly tothe integrating and counting capacitors. For example, in cases where thesignal and/or the background levels fall below a certain threshold(e.g., as indicated by detection signals from one or more detectors),the correlator circuit may be bypassed. This bypass may be achieved, forexample and without loss of generality, by setting the correlation timeto be very high. This bypass may also be achieved by forcing the D inputof the flip flop 1520 of FIG. 15 to be high.

In further embodiments including a single-photon detector-basedreference channel that is configured to detect uncorrelated background(BG) photons for correction of the estimates of the time of flight onthe “main” channel (which is configured to detect an aggregate ofcorrelated signal photons+BG photons), under some operating conditions,the uncorrelated photon count rate, or the signal photon count rate, orboth, may be too high. This can be undesirable in that, if theuncorrelated photon count rate is too high, the probability of detectionof signal photons may be reduced if there is a significant probabilitythat a signal photon will fall/arrive within the single-photondetector's dead time after the arrival of an uncorrelated photon.Conversely, if the signal photon count rate is too high, the probabilityof detection of uncorrelated photons within a dead time after arrival ofa signal photon (and even afterward) may not be uniform, and thus thebackground subtraction/correction operations may provide aninsufficiently accurate estimate of the signal time of arrival. In otherwords, if too many uncorrelated photons are being detected, thesingle-photon detectors may fail to detect signal photons that arrivewithin the detector's subsequent dead time, while if too many signalphotons are being detected, the uncorrelated photons may be detectednon-uniformly. If both of these operating conditions occur, both of theproblems described above may occur. However, in some instances, it maybe undesirable to set a global and/or static limit on the number ofdetected events or photon count rate because the optimal or otherwisedesired number of events for reducing or minimizing the range estimationerror can be a function of the signal photon count level, the backgroundphoton count level, and/or their combination.

In some embodiments, a dynamic event saturation threshold may be used toadjust the number of events in a subframe for a range calculation, forexample to transition the system from the third to the first regimesdescribed above. For example and without loss of generality, the CountSaturation signal that is fed to comparator X2 in the saturation controlcircuit 1455 of FIG. 14 can be adjusted based on the signal andbackground photon count levels measured (i) in a previous cycle, (ii) inanother pixel, or (ii) in both a previous cycle and another pixel. Insome embodiments, a controller outside of the pixel may receive signalsindicative of the number of background and signal+background photoncounts from a controlling cycle, and may set the Count Saturation signallevel based on a lookup table or function, such that the measurementprovides an improved range estimation performance. In some embodiments,the dead time of the SPADs (or other single-photon detectors) in thepixel can be adjusted by controlling the active recharge circuit (e.g.,903), thereby allowing more or fewer avalanches to enter or otherwise beprovided to the correlator circuit (e.g., 925). In some embodiments, thesingle-photon detection probability of the detectors is adjusted, forexample by adjusting the voltage overbias seen by the diodes (e.g.,SPAD1 and SPAD2). Such mechanisms can operate on a per-pixel basis or ongroups of pixels.

That is, in some embodiments of single-photon detectors in dynamicallycontrolled LIDAR applications described herein, the photon count ratemay be adjusted (increased or decreased), for example, by alteringthresholds for saturation control circuits described herein (e.g., basedon signal and background levels from a previous cycle, another pixel, orboth), and/or by altering the dead time of the single photon detectorsby controlling the active recharge circuits described herein. The photoncount rate may be adjusted because, if too many uncorrelated photons arecounted, the detectors may not detect some signal photons that arrivewithin the “dead time” after arrival of a preceding uncorrelated photon,or conversely, if too many signal photons trigger the single-photondetectors, detection of uncorrelated/background levels may benon-uniform, such that background subtraction/correction methodsdescribed herein may not be accurate.

In some embodiments, in-pixel correlation as described herein mayinclude calculation of the center-of-mass of the distribution of therespective times-of-arrival (TOAs) of signal photons over a correlationwindow. Setting the correlation window to be narrow in relation to thepulse width can reduce the number of uncorrelated photons passingthrough the correlator, but may result in loss of or failure to measuresome signal avalanches. Setting the window to be wide in relation to thepulse width can provide for measurement of more avalanches, but mayinclude some background avalanche. That is, in this case moreuncorrelated pairs may be measured. Accordingly, in some embodiments,the duration of the correlation window can be dynamically adjusted (perpixel or per groups of pixels) in response to the measured backgroundand signal count rates.

As discussed herein, some embodiments may operate based on decreasingthe number of uncorrelated avalanches (i.e., responsive to photons thatare uncorrelated to the pulsed laser sources) by reducing the data loadof a single-photon avalanche detector (SPAD) at the point of detection.Such uncorrelated avalanches may result from the SPAD device itself(e.g., due to thermal emissions or tunneling), and/or may result fromthe absorption of photons that do not correspond to the wavelengths ofoptical signals emitted by the LIDAR emitter (such as solar photons, orphotons from other external sources, such as other non-correlatedbackground or ambient light) generally referred to herein as backgroundphotons. In direct time-of-flight systems, this non-correlatedbackground light may have a relatively minor effect on the calculationof the correct photon time-of-flight and hence on the calculation of therange of a target. However, in some embodiments, the effects may be moresevere, for example, for range calculations based the calculation of themean time of arrival (TOA). An effect of uncorrelated avalanches on themeasured time of arrival (TOA) may be a skew towards the center of thetime window over which arrival times are collected.

In the presence of uncorrelated or non-correlated avalanches (alsodescribed herein without loss of generality as background avalanches),the TOA measured by a pixel element (or “pixel”) described herein (whichmay include one or more photodetectors) after integration over manypulse cycles may be:

${TOA}_{meas} = \frac{{\sum\limits_{k = 1}^{m}\left( {t_{{sig},k} \times n_{k}} \right)} + \left( {t_{{win},{mid}} \times n_{bg}} \right)}{n_{{bg} + {sig}}}$

where, k is the index of the time bins within a measurement sequence(e.g., a laser pulse cycle), m is the total number of time bins within asequence, t_(sig,k) is the arrival time of the k^(th) time bin of signalphotons arrival time, with respect to a reference time (heretofore, forsimplicity and without loss of generality taken as the time of the laserpulse excitation (e.g., based on the leading edge of the pulse)), n_(k)is the number of signal (not background) avalanches recorded in thek^(th) time bin, (t_(win,mid)) is the time of or corresponding to thecenter of the time band during which the sequence of avalanches isrecorded, n_(b9) is the total number of background avalanches recordedduring the total integration time and n_(bg+sig) is the total number ofsignal and background avalanches recorded during the integration time.

However, it may be desirable to measure the (mean) signal photons' TOA:

${TOA}_{sig} = \frac{\sum\limits_{k = 1}^{m}\left( {t_{{sig},k} \times n_{k}} \right)}{n_{sig}}$

where n_(sig) is the total number of signal avalanches (avalanchesoriginated by signal photons). In examples described herein, the meantime between avalanches may be relatively large as compared with thedead time of the SPAD such that a change in the number of backgroundavalanches within the measurement range may have no significant effecton the probability of a signal photon to induce an avalanche.

In some embodiments, the avalanche times as referred to herein may betimes of avalanches indicated by detection signals output fromindividual SPAD detectors. In some embodiments, the avalanche times asreferred to herein may be the times of one or both of a correlated pairof avalanches that occur within a correlation time as defined bycorrelator circuits (e.g., 925, 1525) described herein and indicated bycorrelation signals output therefrom.

Further embodiments described herein relate to photon arrival times thatfall or are otherwise detected in two neighboring time bands or distancesub-ranges. Methods and circuits for alternating or otherwise adjustingthe boundaries of the time bands or distance sub-ranges are described.

According to some embodiments described herein, a background-correctedTOA can be calculated by:

${TOA}_{{bg}\; \_ \; {corrected}} = {\frac{\left( {{TOA}_{meas} \times n_{{bg} + {sig}}} \right) - \left( {t_{{win},{mid}} \times n_{bg}} \right)}{n_{sig}} = \frac{\sum\limits_{k = 1}^{m}\left( {t_{{sig},k} \times n_{k}} \right)}{n_{sig}}}$

Some embodiments described herein provide methods and circuits wherebycalculations are performed for correcting errors resulting frombackground avalanches. Pixels described in some embodiments may containan avalanche-counting capacitor or circuit (e.g. 950 a) and atime-integrating capacitor or circuit (e.g., 950 b). These capacitors orcircuits can further be used to count and integrate times ofnon-correlated avalanches by bypassing the in-pixel correlator (e.g.,925).

In some embodiments, a passive acquisition frame may be interspersedbetween one or more active acquisition frames. An active acquisitionframe (also referred to herein as an active frame) may refer to a framein which the pulsed laser of a LIDAR system is active. A passiveacquisition frame (also referred to herein as a passive frame) may referto a frame in which the pulsed laser of the LIDAR system is inactive.

In some embodiments, a passive acquisition SPAD (also referred to hereinas a passive SPAD) or other photodetector may be interspersed in a SPADdetector array such that the passive SPAD is not configured to detectphotons of the wavelength of the emitting laser (i.e., signal photons).In some embodiments, an optical filter may be deposited or otherwiseprovided on top of the active light-receiving area of the passive SPADsuch that the rate of background photons which are transmitted throughthe passive SPAD is proportional to the rate of background photonsimpinging on the active SPADs. In some embodiments, the passive SPAD maybe optically isolated from external illumination, e.g., by a metallayer, and compensation can be made to non-optical backgroundavalanches.

Methods and circuits are described herein with reference to a passiveacquisition frame. It will be understood that similar methods can beused with an active SPAD, whereby correction can be contemporaneous withthe signal acquisition rather than in series or sequence.

Referring again to the circuit of FIG. 9, and without loss ofgenerality, a pixel of a detector array 910 may be operated at aparticular or predetermined frame rate. For example, in some embodimentsthe frame rate may be 10 frames per second. In some embodiments theframe rate may be 20 frames per second. In some embodiments the framerate may be 30 frames per second. During each active frame, a sequenceof laser pulses is emitted (e.g., by VCSEL array 215), and their echoesmay generate avalanches which are processed in the pixel, and in anarray thereof. As an example and without loss of generality, once per apredetermined or desired number of active frames, a passive frame mayreplace an active frame. During the passive frame, the laser drivertransistor/circuitry may not drive the laser to emit pulses.

In some embodiments, each frame may include a number sub-frames. Eachsub-frame spans a part of a complete frame, corresponding to a distancerange that is part of the distance range that can be imaged by the LIDARsystem.

In some embodiments, a partially-passive frame, in which one or moresub-frames are operated in a passive mode, may replace the passive frameof the above description. For example, a partially-passive frame mayreplace an active frame once per a predetermined or desired number ofactive frames. The background counts for other frames may beextrapolated from the measured background count of a passive sub-frame,in proportion to their relative duration with respect to the passivesub-frame. For example, if a passive sub-frame lasts 300 ns and counts100 counts, then another sub-frame which lasts 600 ns, can receive anextrapolated 200 background counts.

Referring to the background-corrected TOA (TOA_(bg) _(_) _(corrected)),the following parameters (a non-exhaustive list) may be processed by theprocessing unit 970. TOA_(meas) is the ratio of integrated time tocounted events at the end of each active sub-frame (scaled by a known ordetermined factor); n_(bg+sig) is the output of the event counter at theend of each sub-frame; t_(win,mid) is the known or determined time ofthe middle of the time band of the current sub-frame; n_(bg) is theoutput of the event counter during a passive sub-frame or anextrapolated count number based on another passive sub-frame, asdescribed above; and n_(sig)=n_(bg+sig) n_(bg) is the calculateddifference between the counter output during each active sub-frame, andthe value of the last passive sub-frame counter for the same pixel,which value may be stored in a non-transitory memory or memory array(for a number of sub-frames) and replaced each time the same sub-frame'spassive output is acquired.

Once parameters have been acquired by the processor unit 970, theprocessor unit 970 may be configured to calculate thebackground-corrected TOA (TOA_(bg) _(_) _(corrected)) in accordance withthe equation above. The processor unit 970 may be implemented by amicrocontroller or microprocessor in some embodiments.

Further embodiments described herein may provide background subtractionin a SPAD pixel, which may differ from embodiments where the signal andbackground are temporally separated on a single pixel based on howmeasurement of the background is divided from the measurement of signal.In particular, rather than measurement of signal and measurement ofbackground separated in time (e.g., during one cycle the emitter isactivated and fires a laser pulse and the data returned is considered tobe “signal” (“S”) or “signal+background” (“S+BG”); during the next cyclethe emitter is disabled and the measurement is considered to be“background only” (“BG”)), further embodiments described herein providea “dual pixel,” whereby a single pixel element comprises two separatedetection channels that operate simultaneously in parallel.

An example dual pixel element 2310 in accordance with embodiments of thepresent disclosure is illustrated in FIG. 23. As shown in FIG. 23,detector element 2310 a defines one of the channels, which is configuredto admit and detect signal+BG photons. Detector element 2310 b definesthe other channel 2310 b, which configured to admit and detect only BGphotons (i.e., so as to exclude detection of the desired signalphotons/wavelength of light). Each of the detector elements 2310 a and2310 b can be implemented as a single detector (e.g., SPAD, APD, etc.)or a combination of detectors, so as to output respective detectionsignals indicative of respective times of arrivals and/or intensities ofphotons incident thereon. In some embodiments, the detector elements2310 a and 2310 b in FIG. 23 (which may be considered “subpixels” of theillustrated pixel element 2310) can be implemented as a pair of SPADs,which may produce/detect correlated avalanche events in accordance withthe embodiments discussed above.

Distinguishing between the two simultaneously operating channels S+BGand BG defined by detector elements 2310 a and 2310 b, respectively, maybe performed by various methods and devices. Some embodiments describedherein may use optical bandpass filters, where the detector 2310 a forone channel S+BG includes a bandpass filter that is open (i.e., isconfigured to permit light) at the wavelength of the emitter andtherefore allows signal photons to pass through; and the detector 2310 bfor another channel BG includes a different optical bandpass filter(illustrated in FIG. 23 as optical filter 2312) that is configured toblock the passage of signal photons (i.e., has a transmission band thatis configured to filter photons/light having wavelengths output from theemitter), but allow non-signal background photons (e.g., photons frombackground or ambient light sources, including the sun) to pass in aquantity that is a linearly related to the quantity of backgroundphotons that would be expected to pass through the other channel S+BG(in addition to the signal). By operating these two channels S+BG and BGsimultaneously, a simultaneous collection of both signal and backgroundphotons may be realized.

Some advantages of the embodiment of FIG. 23 over temporal multiplexingbetween signal and background may include (but are not limited to): anincrease in integration time of the signal (because no time is “wasted”switching to “only BG” collection); and simultaneous sampling of S+BGand BG allows for other electrical embodiments whereby analog signalsfrom each channel could be subtracted in real time. Some disadvantagesmay include (but are not limited to): additional “real estate” or areaper pixel (e.g., a larger footprint) in the focal plane, which mayreduce pixel count (for a same size array) or increase area of the array(to achieve the same pixel count); different optical bandpass filters tobe applied to either or both of the two sub-pixels. The differentoptical bandpass filters may be implemented using micro-transferprinting techniques in some embodiments. In further embodiments,different optical bandpass filters may be implemented by standardoptical filter deposition and growth approaches in combination withphotolithographic masking methods and processes.

FIG. 24A and FIG. 24B illustrate operations for phase shifting tocorrect for a distribution of detected photons that may span twosubframes in accordance with embodiments of the present disclosure. Inparticular, in some instances, the laser signal echoes reflected from atarget may span two subframes. Each subframe may span a part of acomplete frame, with each complete frame corresponding to a distancerange being imaged. Each subframe may also include data for a respectivestrobe window of the detector array, with each strobe windowcorresponding to a respective sub-range of the distance range beingimaged. A histogram of such arrivals (e.g., photon arrivals) is shown inFIG. 24A, where the horizontal axis represents the bins of time from orrelative to a reference time (e.g., laser emission/firing), and canlikewise represent bins of calculated range from a LIDAR system. Thevertical axis represents the number of counts (e.g., photon counts)collected in each time bin (or sub-range of the distance range beingimaged).

The time-distribution of times-of-arrival (TOAs) can extend beyond thelimit of a sub-frame, where each subframe includes data for one strobewindow. Because a pixel (which may include one or more detectors) asdescribed in some embodiments herein may not output a histogram, butrather, a scalar value indicating the average over all integratedavalanche times, such a distribution can result in an erroneousestimation of the actual mean of arrival times. Some embodimentsdescribed herein provide methods and circuits that may alleviate sucherrors.

In particular, a timing circuit (which may be implemented by any of thecontrol circuits described herein, e.g., 105, 205) is operable tocontrol the subframe or sub-range limits of the detector array (e.g.,110, 210), for example, a SPAD-based or other detector array, forexample, by altering timing and/or durations of the corresponding strobewindows. These time limits specify when the SPAD devices of the detectorarray are charged/activated to detect photons and generate detectionsignals responsive to avalanche events. In some embodiments, analternating pattern of subframe limits can be employed to addressinstances in which the echoes from a single target span two subframes.In some embodiments, the time limits for subframes of respectivedetectors may be controlled by altering a duration/strobe window forwhich respective detectors of the array are activated, for example,using a time gating scheme, including but not limited to providingdifferent ranges for different detectors at different positions of thearray.

In the example of FIG. 24A and FIG. 24B, a target may be at a range of161 meters (m) from the array, and the echoes may be distributed totimes corresponding to ranges of 159 m to 163 m. A first frame mayinclude a first subframe corresponding to a range of 150 m-160 m, and asecond subframe corresponding to 160 m-170 m. The pixel may calculate afirst target around 159 m for the first sub-frame, and (as shown in FIG.24A) a second target around 162 m in the second sub-frame; however, bothof these calculations are erroneous.

According to some embodiments described herein, a phase-shifted sequenceof sub-frames is driven by the timing circuit, which may provide strobesignals that alter the timing and/or durations of the strobe windows ofthe detector array. In this example, the phase-shifted frames includesubranges of 155 m-165 m and 165 m-175 m, such that no target iscalculated for the first subframe (corresponding to the range of 150m-160 m) and a target is calculated for the second subframe(corresponding to 160 m-170 m) at the correct range of 161 m (as shownin FIG. 24B).

In some embodiments, the timing circuitry is configured to continuouslyalternate or otherwise adjust the subframe boundaries. In someembodiments, the timing circuitry is configured to vary the subframeboundaries from frame to frame. In some embodiments, the timingcircuitry is configured to vary the sub-frame boundaries if (and/or onlyif) targets are identified in adjacent sub-frames.

In some embodiments, methods of alternating sub-frame boundaries may beused to distinguish spurious noise from desired optical signals emittedby the emitter array. By way of explanation, if a background or ambientlight levels are perfectly uniform, the measured TOA would be the centerof the sub-frame. Methods of alternating boundaries as described hereinmay address (but are not limited to) the following scenarios which canlead to errors. In one example, a target may be located at the center ofthe subframe, e.g., at a distance that is equally spaced from the boundsof the range covered by the subframe (e.g., at a distance of 165 m inthe example second subframe above), which could be construed as aperfectly symmetrical noise, and detection of the target might thus bemissed by the LIDAR. Upon alternating to a phase-shifted frame inaccordance with embodiments described herein, two targets may beidentified (one at the 155 m-165 m phase-shifted sub-frame, and anotherat the 165 m-175 m phase-shifted sub-frame), and a processing unit thatreceives the detector output can infer the correct position of thetarget. Similarly, the noise may not be perfectly symmetrical and afalse target reading may result, for example, indicating that a targetis present at a distance of 166 m. During the phase-shifted frame inaccordance with embodiments described herein, since no target isdetected in the 155 m-165 m sub-frame and a stronger bias may berequired to result in a mean range of 166 m in the 165 m-175 msub-frame, the first reading can be tagged or otherwise identified bythe processing unit as false.

As discussed above with reference to FIGS. 1-3, some embodimentsdescribed herein are directed to methods of fabricating and operating atunable optical filter (e.g., tunable filter 212, 312), whose pass bandcan track or otherwise be varied to correspond to the light emissionband or wavelength range of one or more LIDAR emitters (e.g., of arrays115, 215), for example, in response to a change in the emitters'operating temperature (e.g., as indicated by temperature monitor 213).Tuning of the variable optical filter may be implemented by actuation ofone or more actuator elements (e.g., 344) by a control circuit. In someembodiments, controlling the tilting of the optical filter to provide adesired tilt angle in response to actuation of the one or more actuatorelements may affect or alter the optical transmission characteristics ofthe filter. For example, the filter may be configured to transmitshorter wavelengths of light as the tilt angle increases relative to thenormal angle of incidence.

In some embodiments, as shown in FIGS. 25A and 25B, the variable ortunable optical filter 2512 may be controlled by actuators, such aspiezoelectric actuators. For example, in FIG. 25A, an optical narrowbandfilter 2512 is mechanically-coupled to one or more electrodes 2507,2511. In the example of FIG. 25A, the filter 2512 is mounted on a rigidsubstrate (e.g., a printed circuit board (PCB)) 2508, 2510 that includesthe electrodes 2507, 2511 thereon at opposite sides of the filter 2512.Alternatively, the one or more electrodes 2507, 2511 may be deposited onthe filter 2512 itself, for example by sputtering, by chemical vapordeposition, or other deposition methods.

In some embodiments, a substrate 2502, 2504 includes at least oneelectrode 2501, 2501′, 2506, 2506′ on a surface thereof facing theelectrodes 2507, 2511. An impedance measurement circuit 2509 is coupledto two of the electrodes 2501′, 2506′, and a voltage driving circuit2503 is coupled to the impedance-measurement circuit 2509. The voltagedriving circuit 2503 provides a radio-frequency (RF) voltage signal totwo of the substrate electrodes 2501, 2506. The RF signal iscapacitively coupled to the floating PCB electrodes 2507, 2511 and backto substrate electrodes 2501′, 2506′. The impedance measurement circuit2509 measures the impedances across both paths (for example, 2501→2511and 2511→2501′, or 2506→2507 and 2507→2506′).

In some embodiments, mounted filter 2512 is attached to a substrate2502, 2504 through at least one actuator 2544. For example as shown inFIG. 25B, the mounted filter 2512 may be attached to the substrate 2502,2504 through two piezoelectric crystals as the actuators 2544, which maybe controlled by a piezoelectric drive 2511 based on the impedancemeasurements from the impedance measurement circuit 2509.

A control circuit 2505 may receive as input the two measured impedancesfrom the impedance measurement circuit 2509, a temperature measurementfrom the emitter array (e.g., from temperature monitor 213), and datafrom a calibration table (having entries for impedance as a function ofangle). The control circuit 2505 generates voltages to drive the anelectrostatic force between pairs of electrodes 2501, 2511/2506,2507 oractuate piezoelectric stages 2544 such that the transmission spectrum ofthe filter 2512 tracks or corresponds to the emission spectrum of theemitter.

In some embodiments, a calibration process may be performed to generatelookup tables. For example, in some embodiments, a sequence of voltagesmay be applied to the piezoelectric elements 2544 while a broadbandcollimated source illuminates the filter. A spectrometer on the oppositeside of the filter may measure the transmitted spectrum. The impedancemeasurement circuit 2509 may measure the impedance at either side of thefilter 2512. Using the formula for transmission wavelength λ(θ)dependence on angle of incidence θ, where λ₀ is wavelength at normalincidence and n_(eff) is the effective index of refraction,

λ(θ)=λ₀√{square root over (1−(sin θ/n _(eff))²)}

a table may be generated, including measured impedances as a function ofeach tilt angle. In some embodiments, this calibration may be conductedfor a plurality of temperatures or temperature ranges. In someembodiments, a temperature correction factor may be applied to theimpedance measurement in order to fit to the correct or correspondingtilt angle. In some embodiments the calibration table is stored in amemory that is accessible to the control circuit 2505.

In some embodiments, the temperature dependence of emission wavelengthmay be known, e.g., 0.08 nm per degree Celsius. In some embodiments,during operation, a temperature sensor (e.g., 213) may measure thetemperature of the emitter array and may transmit this information tothe control circuit 2505 or other processing unit. The processing unitmay determine the set of impedances corresponding to the tilt anglewhich matches with the measured emitter temperature. The processing unitmay also receive inputs from the impedance measurement circuits 2509,and may determine whether the drive voltage to either of the piezoactuators 2544 should be increased or decreased. Operations may continueuntil the desired impedance values are reached.

In some embodiments, actuation may be implemented via magnets instead ofpiezoelectric elements, and the magnitude of the tilt force can becontrolled by adjusting a current through coils on the substrate.

In some embodiments, actuation may be implemented via an electrostaticforce between pairs of electrodes acting as capacitors. For example andwithout loss of generality, the electrodes 2507, 2511 on the filter PCB2508, 2510 may be negatively charged. When the distance between a pairof electrodes on one side of the filter 2512 should be increased toachieve a desired tilt, negative charge is applied to the substrateelectrode(s) 2501 or 2506 on that side of the filter 2512, or viceversa.

In some embodiments, actuation may be implemented mechanically. Forexample, rotation of a screw may be utilized to push or pull one side ofthe filter 2512 in order to effect the desired tilt.

Further embodiments described herein are directed to tuning the opticalpass band of a variable or tunable optical filter (e.g., tunable filter212, 312) by temperature tuning. In particular, rather than (or inaddition to) varying the mechanical position of the optical pass bandfilter, the temperature of the filter may be varied to affect or alterthe optical transmission characteristics of the filter. In order torealize such a system, the temperature coefficient of the optical passband may be characterized and known (for example, via a calibrationprocess) such that the desired temperature of the variable opticalfilter can be set and varied in accordance with variations of thewavelength of the emitter. Optical filters typically have some, albeitsmall, dependence on temperature. This dependence can be complex and mayresult from various effects including (but not limited to) changes indensity and refractive index of the materials in the thin film filter,as well as changes in the physical thickness of the layers (e.g., thethin film layers) in the stack resulting from thermal expansion. Thetemperature coefficient of many dielectric filters may be less than thetemperature coefficient of the emitter (e.g. for VCSELs, 0.06nm/degree), such that a temperature tuned optical bandpass filter may bebased on dielectric stacks.

Another approach to provide temperature tunable optical filters inaccordance with embodiments described herein may be to incorporate thesame material set used for the emitters (e.g., the same materials as anoutput coupler distributed Bragg reflector (DBR) of emitter VCSELs) asthe bandpass filter on the receiver/detector. For example, in someembodiments, a temperature tunable optical filter may be an AlGaAs/GaAsDBR that is deposited on top of a GaAs substrate (with bandgap largeenough to allow transparency to desired photon wavelengths, e.g. 940nm). Because the same materials and designs are used in thistemperature-tuned bandpass filter (e.g., with respect to the emitteractive region), the characteristics of the optical filter will vary withtemperature in the same way as the emitter. Some embodiments describedherein may thermally couple the two (emitter and receiver/detectorbandpass filter) in a passive way, such that the temperature of thevariable optical bandpass filter can be maintained equal to (or at aconstant offset relative to) that of the emitter, and the pass band ofthe filter on the receiver/detector would vary with the wavelength ofthe emitter.

Further embodiments relating to correlation and background correctionare described below with reference to FIG. 22. In particular, thecorrelator circuit may be configured to generate a correlation signalonly when a pair of avalanche events occur within the predeterminedcorrelation time or time window specified by the correlator circuit. Asshown in FIG. 22, embodiments herein may be used to distinguish betweentwo types of correlation signal outputs: those that result fromdetection signals generated responsive to at least one of the inputavalanches being a signal photon “s” (designated a or “alpha”) and thosewhich result from detection signals generated responsive to inputavalanche event pairs that result only from background photons“b”(designated (3 or “beta”).

The calculated average time of arrival may be polluted by detection ofbackground photons. The calculated average time of arrival over thecorrelator outputs (t_(meas1)) may be a random variable (R.V.) that is afunction of other random variables:

$\begin{matrix}{{t_{{meas}\; 1} = \frac{\left( {\sum_{i}^{N_{\alpha}}{t_{\alpha}(i)}} \right) + \left( {\sum_{i}^{N_{\beta}}{t_{\beta}(i)}} \right)}{N_{\alpha} + N_{\beta}}}{t_{{meas}\; 1} = \frac{T_{\alpha} + T_{\beta}}{N_{\alpha} + N_{\beta}}}} & \left( {{Eq}.\mspace{11mu} 1} \right)\end{matrix}$

Each of the variables in t_(meas1) may be a random variable. Determiningthe distribution of t_(meas1) can be performed by: (1) computingt_(meas1) repeatedly in a monte carlo fashion; (2) algebraicallycombining the analytical expressions for the distributions of each ofthe four R.V. on the right hand side (RHS) of Eq. 1. Expressions for thedistributions of the variables on the RHS are provided for both of thesealternatives below.

One way to model the distributions for each of these random variables iswith a Gaussian normal distribution. A basis for doing so is the centrallimit theorem, which indicates that a distribution which will govern thesum of any large number of random variables is a Gaussian. Theconditions of the central limit theorem may generally be fulfilled inthe random variables above. The mean value of the distribution for eachrandom variable may be:

$\begin{matrix}{{\overset{\_}{N_{\alpha}} = {p_{\alpha} \times N_{opp}}}{\overset{\_}{N_{\beta}} = {p_{\beta} \times N_{opp}}}{\overset{\_}{T_{\alpha}} = {\left( {t_{target} + \frac{t_{p}}{2}} \right) \times N_{\alpha}}}{\overset{\_}{T_{\beta}} = {\frac{T_{gate}}{2} \times N_{\beta}}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

where t_(target)=the actual time of flight; t_(p)=pulse width; andp_(α), p_(β), and N_(opp) are defined below. Eq. 2 above provides halfthe parameters to define Gaussian distributions (the expectationvalues). As it may be difficult is to determine what the standarddeviation for these Gaussian distributions should be, the descriptionbelow looks at each random variable individually to determine theanalytic function governing its distribution.

Using the random variables N_(α) and N_(β), which are the total numberof correlator outputs (within a given strobe gate) after integrationover one frame corresponding to case α or β as described above, theprobability distribution of some number of events occurring may be aBernoulli process assuming that the probabilities for each individualevents are “IID” (independent and identically distributed). Theprobability of a “beta output” of the correlator can be similarlyconsidered: over the course of integration there may be N_(opp)=#opportunities for the correlator to output a correlation signal inregard to the beta event occurring. In other words, over the time spanof one time window on the correlator, there may be two possibilities: itcan report a beta correlation event or not. The number of opportunitiesfor a correlator output, i.e. N_(opp), should be equal to the totalintegration time divided by the time window duration:

$\begin{matrix}{{N_{{opp},\beta} = {\left( \frac{T_{gate}}{t_{win}} \right) \times \left( {\# \mspace{11mu} {dwells}\mspace{14mu} {on}\mspace{14mu} {gate}\mspace{14mu} {per}\mspace{14mu} {frame}} \right)}}{N_{{opp},\beta} = {\left( \frac{T_{gate}}{t_{win}} \right) \times \left( {\frac{T_{frame}}{T_{{laser}\mspace{11mu} {cycle}}}\frac{1}{N_{gates}}} \right)}}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

If the conditions for a Bernoulli process are fulfilled by thisinterpretation of the correlator output, then the formula for theprobability distribution of the number of Bernoulli successes can beused to describe the total number of beta outputs of the correlator:

$\begin{matrix}{{{PDF}_{gate}\left( N_{\beta} \right)} = {\begin{pmatrix}N_{opp} \\N_{\beta}\end{pmatrix} \times p_{\beta}^{N_{\beta}} \times \left( {1 - p_{\beta}} \right)^{N_{opp} - N_{\beta}}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

where the probability of a beta output at each opportunity is given bythe Poisson probability of two (or more, k>=2) events occurring duringthe time window:

p _(β) =p _(b)(k≥2; n _(b) )=1−p _(b)(k=1; n _(b) )−p _(b)(k=0; n _(b))p _(β) =p _(b)(k≥2; n _(b) )=1−( n _(b) +1)×e n_(b)   (Eq. 5)

where the expected number of avalanche arrivals (resulting from BGphotons) during the time window is the product of the average bg photonarrival rate, b, and the duration of the correlator time window,t_(win):

n _(b) =b×t _(win)  (Eq. 6)

The beta outputs (i.e. coincidences between two BG photons only) areaddressed in the above examples. For the alpha outputs, it may bere-examined how many opportunities there are and how the probability ofa correlator output is different for alpha outputs, which consider thecombination of signal and background.

Rather than focus only on alpha outputs (i.e. s+s, s+b, b+s only) andexclude beta outputs (b+b), some embodiments may compute the probabilitydistribution for all possible correlator outputs, and then subtract theprobability associated with only beta outputs.

In focusing on total number of events when the signal plays a role, thenumber of possible events may be restricted to only the time within thereturn of the pulse echo (i.e. inside the pulse width only). In someexamples, only the case where the correlation time window is shorterthan the pulse duration (t_(win)<t_(p)) may be considered. In this case,there may be (t_(p)/t_(win)) of these opportunities per gate period(assuming a pulse is in the gate). Therefore, the number ofopportunities (for either alpha or beta output) is now:

$\begin{matrix}{N_{{opp}{({\alpha + \beta})}} = {\left( \frac{t_{pulse}}{t_{win}} \right) \times \left( {\frac{T_{frame}}{T_{{laser}\mspace{11mu} {cycle}}}\frac{1}{N_{gates}}} \right)}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

Likewise, the probability of a correlation signal during one of theseopportunities follows the same form as above,

p _(α+β) =p _(s+b)(k≥2; n _(s+b) )=1−( n _(s+b) +1)×e n_(s+b)

but the expected number of photon arrivals during this opportunity maybe higher according to the higher rate of signal photons arriving:

n _(s+b) =(s+b)×t _(win)  (Eq. 8)

The probability distribution for number of (alpha or beta) correlatoroutputs that result from the time period of the pulse only, then, maybe:

$\begin{matrix}{{{PDF}_{p}\left( N_{\alpha + {\beta \; p}} \right)} = {\begin{pmatrix}N_{{opp}\; {({\alpha + \beta})}} \\N_{\alpha + {\beta \; p}}\end{pmatrix} \times p_{\alpha + \beta}^{N_{\alpha + {\beta \; p}}} \times \left( {1 - p_{\alpha + \beta}} \right)^{N_{{opp}\; {({\alpha + \beta})}} - N_{\alpha + {\beta \; p}}}}} & \left( {{Eq}.\mspace{14mu} 9} \right)\end{matrix}$

where β_(p) refers to only beta correlations that occur during thepulse, as opposed to any other time during the gate.

The PDF (probability density function) for the number of alpha outputsonly may be desired, and an expression for this may be determined bycomputing the PDF for the number of beta events that occur during thepulse only, N_(βp).

$\begin{matrix}{{{PDF}_{p}\left( N_{\beta \; p} \right)} = {\begin{pmatrix}N_{{opp}\; {({\alpha + \beta})}} \\N_{\beta \; p}\end{pmatrix} \times p_{\beta}^{N_{\beta \; p}} \times \left( {1 - p_{\beta}} \right)^{N_{{opp}\; {({\alpha + \beta})}} - N_{\beta \; p}}}} & \left( {{Eq}.\mspace{14mu} 10} \right)\end{matrix}$

Combining these two functions may yield:

${{PDF}\left( N_{\alpha} \right)} = {{{PDF}\left( {N_{\alpha + {\beta \; p}} - N_{\beta \; p}} \right)} = {\begin{pmatrix}N_{{opp}\; {({\alpha + \beta})}} \\{N_{\alpha + {\beta \; p}} - N_{\beta \; p}}\end{pmatrix} \times p_{\beta}^{N_{\alpha + {\beta \; p}} - N_{\beta \; p}} \times \left( {1 - p_{\beta}} \right)^{N_{{opp}\; {({\alpha + \beta})}} - {({N_{\alpha + {\beta \; p}} - N_{\beta \; p}})}}}}$

The random variable T_(β) may refer to the sum of all beta correlatoroutputs which result from coincidences of background photons within thecorrelation time window of the correlator. The distribution of theseevents may be uniformly distributed across the duration of the gate, 0 .. . T_(gate). The expectation value for the uniformly distributed randomvariables, t_(β)(i), is ½ T_(gate). The distribution may be the “IrwinHall” distribution or the uniform sum distribution. Substitutingappropriately, the expression for the distribution of possible valuesabout the expected value (½ Tgate) may be:

$\begin{matrix}{{{PDF}_{T_{\beta}}\left( {T_{\beta};N_{\beta}} \right)} = {\frac{1}{2{\left( {N_{\beta} - 1} \right)!}}{\sum\limits_{k = 0}^{N_{\beta}}\; {\left( {- 1} \right)^{k}\begin{pmatrix}N_{\beta} \\k\end{pmatrix}\left( {\frac{T_{\beta}}{T_{gate}} - k} \right)^{n - 1}{{sgn}\left( {\frac{T_{\beta}}{T_{gate}} - k} \right)}}}}} & \left( {{Eq}.\mspace{14mu} 11} \right)\end{matrix}$

This distribution may depends on the number of instances, N_(β), of therandom variable, t_(β)(i) that contribute to the sum, T_(β). The largerN_(β) the tighter the distribution and the more likely that itapproximates the expectation value.Furthermore, T_(β) and N_(β) may be “linked.” For example, if aparticular value is used for N_(β), (e.g., 113), then the exact samenumber (113) may be used when generating the value for T_(β) (i.e.,N_(β) may not be re-generated for the purposes of generating T_(β)).References for distribution of sum & mean of uniformly distributedrandom variables:

Irwin Hall (sum):

$X = {{\sum\limits_{k = 1}^{n}\; {U_{k}.\mspace{14mu} {{fx}\left( {x;n} \right)}}} = {\frac{1}{2{\left( {n - 1} \right)!}}{\sum\limits_{k = 0}^{n}\; {\left( {- 1} \right)^{k}\begin{pmatrix}n \\k\end{pmatrix}\left( {x - k} \right)^{n - 1}{{sgn}\left( {x - k} \right)}}}}}$

Bates (average):

$X = {{\frac{1}{n}{\sum\limits_{k = 1}^{n}\; {U_{k}.\mspace{14mu} {{fx}\left( {x;n} \right)}}}} = {\frac{n}{2{\left( {n - 1} \right)!}}{\sum\limits_{k = 0}^{n}\; {\left( {- 1} \right)^{k}\begin{pmatrix}n \\k\end{pmatrix}\left( {{nx} - k} \right)^{n - 1}{{sgn}\left( {{nx} - k} \right)}}}}}$

For the random variable T_(α) (assuming that thet_(window)<<t_(pulse)=t_(p)), correlator events which result from thecoincidence of at least one signal photon avalanche, by definition,occur within the time of the pulse duration:t_(target)<t_(α)(i)<t_(targ)+t_(pulse). For the duration of the (square)pulse, it may be assumed that the likelihood of all values within thisdomain is uniform. Thus, the problem may be considered as identical tothe one for Tb or T_(β), with a difference being that the time domainover which correlator events may occur is different: roughly,T_(gate)→T_(pulse) Additionally the number of events expected may bedifferent, N_(β) →N_(α). Making these substitutions, a PDF which isdefined only on the interval (0 . . . t_(pulse))+t_(target) may bederived:

$\begin{matrix}{{{PDF}_{T_{\alpha}}\left( {T_{\alpha};N_{\alpha}} \right)} = {\frac{1}{2{\left( {N_{\alpha} - 1} \right)!}}{\sum\limits_{k = 0}^{N_{\alpha}}\; {\left( {- 1} \right)^{k}\begin{pmatrix}N_{\alpha} \\k\end{pmatrix}\left( {\frac{T_{\alpha}}{t_{p}} - k} \right)^{N_{\alpha} - 1}{{sgn}\left( {\frac{T_{\alpha}}{t_{p}} - k} \right)}}}}} & \left( {{Eq}.\mspace{14mu} 12} \right)\end{matrix}$

This distribution may be defined only within the pulse duration, and maybe zero otherwise. Further, the larger that Na is the closer the PDFapproximates a delta function centered on t_(targ)+½t_(p). Given theseconsiderations, the limiting case may be considered:

$\begin{matrix}{{\lim\limits_{N_{\alpha}\rightarrow{large}}T_{\alpha}} = {t_{targ} + {\frac{1}{2}t_{p}}}} & \left( {{Eq}.\mspace{14mu} 13} \right)\end{matrix}$

In implementing background correction in accordance with someembodiments described herein, the expression (Eq. 1) describes a randomvariable that represents a given measurement of the average time ofarrival of correlator output events in the presence of both signal andbackground. Further, the random variables which make up the right handside of Eq. 1 have been described with analytic expressions for theirprobability distributions. The time of arrival of signal photons “s” isdesignated t_(targ) in the expressions above and is measured as a valuewithin the domain 0 . . . T_(gate).

To recover the value of t_(targ) given the knowledge of each of theother quantities in Eq. 1, the limit described in Eq. 13 can besubstituted into Eq. 1 for Ta (or T_(α)):

$\begin{matrix}{t_{{meas}\; 1} = {\frac{T_{\alpha} + T_{\beta}}{N_{\alpha} + N_{\beta}} = \frac{\left( {t_{targ} + {\frac{1}{2}t_{p}}} \right) + T_{\beta}}{N_{\alpha} + N_{\beta}}}} & \left( {{Eq}.\mspace{14mu} 14} \right)\end{matrix}$

rearranging to isolate the desired quantity:

$\begin{matrix}{t_{targ} = {{\left( {N_{\alpha} + N_{\beta}} \right)t_{{meas}\; 1}} - T_{\beta} - {\frac{1}{2}t_{p}}}} & \left( {{Eq}.\mspace{14mu} 15} \right)\end{matrix}$

Combination of multiple random variables with their PDFs can beaccomplished by considering error propagation models. Eq. 1 (or Eq. 14)may be used to derive a representation of the PDF for t_(meas1) on theright hand side. Likewise, the distribution of actual outcomes fort_(targ) (in Eq. 15) may be determined. In some embodiments, BGcorrection based on analysis above may be implemented as follows:

-   -   1. Select a “true” value for t_(targ), t_(targ,true).    -   2. Select parameter values:        -   a. t_(p)=duration of pulse; Tgate=duration of gate;        -   b. s=rate of signal photon arrivals (during the pulse only)        -   c. b=rate of background photon arrivals (over entire gate)        -   d. T_(integ)=time of integration on this pixel. In case of            flash LIDAR set this to the frame duration        -   e. N_(gates)=number of strobe gates that have to share the            total integration time.    -   3. compute PDFs functions    -   4. Begin a single iteration (e.g., of monte carlo analysis) for        given integration time. Note: a single iteration may have a        determined number of laser cycles and value for N_(opp), etc.        This single iteration may generate a unique set of values for:        -   a. N_(α)        -   b. N_(β)        -   c. T_(β)        -   d. And t_(meas1)=Eq. 14.    -   5. Repeating the iterations in step 4 may yield a distribution        for t_(meas1)    -   6. For each of the iterations in step 4 that are used to compute        t_(meas1) a second set of background values can be computed        (indicated by a prime):        -   a. N′_(β)        -   b. T′_(β)    -   7. The error for each iteration can be computed by:

${error} = {\left( {t_{targ} - t_{{targ}_{true}}} \right) = {{\left( {N_{\alpha} + N_{\beta}^{\prime}} \right)t_{{meas}\; 1}^{montecarlo}} - T_{\beta}^{\prime} - {\frac{1}{2}t_{p}}}}$

where t_(meas1) (monte carlo) on the RHS is from step 4 but the otherquantities are from step 6.

Accordingly, embodiments described herein provide integrated solid-statesystems that can identify and localize objects in three dimensions andat varying sunlight conditions. In some embodiments, the system includesa pulsed light source, a detector with an array of single-photonavalanche detectors (SPADs) and on-chip data reduction or minimizationcircuitry, and a control and processing unit. A substrate with at leastone laser source is driven to emit a train of pulses which illuminate awide and deep field of view. Reflected optical signals are filtered toremove ambient light and trigger avalanches in individual pixels in theSPAD array. Processing circuitry reduces the amounts of generated datato a voltage corresponding to the distance between a target and thesensor for a given azimuth and altitude. A processing unit generates athree-dimensional point cloud.

Some embodiments described herein may be applied to LIDAR systems foruse in, for example, ADAS (Advanced Driver Assistance Systems),autonomous vehicles, UAVs (unmanned aerial vehicles), industrialautomation, robotics, biometrics, modeling, augmented and virtualreality, 3D mapping, and security. In some embodiments, the emitterelements of the emitter array may be vertical cavity surface emittinglasers (VCSELs). In some embodiments, the emitter array may include anon-native substrate having thousands of discrete emitter elementselectrically connected in series and/or parallel thereon, with thedriver circuit implemented by driver transistors integrated on thenon-native substrate adjacent respective rows and/or columns of theemitter array, as described for example in U.S. Provisional PatentApplication No. 62/484,701 entitled “LIGHT DETECTION AND RANGING (LIDAR)DEVICES AND METHODS OF FABRICATING THE SAME” filed Apr. 12, 2017, andU.S. Provisional Patent Application No. 62/613,985 entitled “ULTRA-SMALLVERTICAL CAVITY SURFACE EMITTING LASER (VCSEL) AND ARRAYS INCORPORATINGTHE SAME” filed Jan. 5, 2018, with the United States Patent andTrademark Office, the disclosures of which are incorporated by referenceherein.

Various embodiments have been described herein with reference to theaccompanying drawings in which example embodiments are shown. Theseembodiments may, however, be embodied in different forms and should notbe construed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure is thorough andcomplete and fully conveys the inventive concept to those skilled in theart. Various modifications to the example embodiments and the genericprinciples and features described herein will be readily apparent. Inthe drawings, the sizes and relative sizes of layers and regions are notshown to scale, and in some instances may be exaggerated for clarity.

The example embodiments are mainly described in terms of particularmethods and devices provided in particular implementations. However, themethods and devices may operate effectively in other implementations.Phrases such as “example embodiment”, “one embodiment” and “anotherembodiment” may refer to the same or different embodiments as well as tomultiple embodiments. The embodiments will be described with respect tosystems and/or devices having certain components. However, the systemsand/or devices may include fewer or additional components than thoseshown, and variations in the arrangement and type of the components maybe made without departing from the scope of the inventive concepts. Theexample embodiments will also be described in the context of particularmethods having certain steps or operations. However, the methods anddevices may operate effectively for other methods having differentand/or additional steps/operations and steps/operations in differentorders that are not inconsistent with the example embodiments. Thus, thepresent inventive concepts are not intended to be limited to theembodiments shown, but are to be accorded the widest scope consistentwith the principles and features described herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It also will be understood that, as used herein,the term “comprising” or “comprises” is open-ended, and includes one ormore stated elements, steps and/or functions without precluding one ormore unstated elements, steps and/or functions. The term “and/or”includes any and all combinations of one or more of the associatedlisted items.

Spatially relative terms, such as “beneath,” “below,” “bottom,” “lower,”“above,” “top,” “upper,” and the like, may be used herein for ease ofdescription to describe one element's or feature's relationship toanother element(s) or feature(s) as illustrated in the Figures. It willbe understood that the spatially relative terms are intended toencompass different orientations of the device in use or operation inaddition to the orientation depicted in the Figures. For example, if thedevice in the Figures is turned over, elements described as “below” or“beneath” other elements or features would then be oriented “above” theother elements or features. Thus, the term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations), and the spatiallyrelative descriptors used herein may be interpreted accordingly. Inaddition, it will also be understood that when a layer is referred to asbeing “between” two layers, it can be the only layer between the twolayers, or one or more intervening layers may also be present.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent inventive concept.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. In no event, however, should “on” or“directly on” be construed as requiring a layer to cover an underlyinglayer.

Embodiments are described herein with reference to cross-sectionaland/or perspective illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures). As such, variationsfrom the shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are to be expected. Thus,embodiments should not be construed as limited to the particular shapesof regions illustrated herein but are to include deviations in shapesthat result, for example, from manufacturing. For example, an implantedregion illustrated as a rectangle will, typically, have rounded orcurved features and/or a gradient of implant concentration at its edgesrather than a binary change from implanted to non-implanted region.Likewise, a buried region formed by implantation may result in someimplantation in the region between the buried region and the surfacethrough which the implantation takes place. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the actual shape of a region of a device andare not intended to limit the scope of the present inventive concepts.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this inventive concept belongs. Itwill be further understood that terms, such as those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Many different embodiments have been disclosed herein, in connectionwith the above description and the drawings. It will be understood thatit would be unduly repetitious and obfuscating to literally describe andillustrate every combination and subcombination of these embodiments.Accordingly, the present specification, including the drawings, shall beconstrued to constitute a complete written description of allcombinations and subcombinations of the embodiments described herein,and of the manner and process of making and using them, and shallsupport claims to any such combination or subcombination.

In the drawings and specification, there have been disclosed embodimentsof the disclosure and, although specific terms are employed, they areused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the present invention being set forth in thefollowing claims.

That which is claimed:
 1. A Light Detection And Ranging (LIDAR)apparatus, comprising: a pulsed light source configured to emit opticalsignals; a detector array comprising single-photon detectors that areconfigured to output respective detection signals indicating respectivetimes of arrival of a plurality of photons incident thereon, wherein thephotons comprise signal photons having wavelengths corresponding to theoptical signals from the pulsed light source and background photonshaving wavelengths corresponding to at least one other light source; andprocessing circuitry configured to receive the respective detectionsignals output from the single-photon detectors, wherein the processingcircuitry comprises one or more of: a recharge circuit configured toactivate and deactivate subsets of the single-photon detectors forrespective strobe windows between pulses of the optical signals and atrespective delays that differ with respect to the pulses, responsive torespective strobing signals; a correlator circuit configured to outputrespective correlation signals representing detection of one or more ofthe photons whose respective time of arrival is within a predeterminedcorrelation time relative to that of at least one other of the photons;a time processing circuit comprising a counter circuit configured toincrement a count value responsive to the respective correlation signalsor detection signals, and a time integrator circuit configured togenerate an integrated time value based on the respective times ofarrival indicated by the respective correlation signals or detectionsignals with respect to a reference timing signal, wherein a ratio ofthe integrated time value to the count value indicates an average timeof arrival of the photons.
 2. The LIDAR apparatus of claim 1, furthercomprising: a tunable optical filter element arranged to transmit thephotons that are incident on the detector array, the tunable opticalfilter element having a transmission band that is configured to varybased on a spectrum of the optical signals output from the pulsed lightsource, a temperature of the pulsed light source, and/or a temperatureof the tunable optical filter element.
 3. The LIDAR apparatus of claim1, wherein the processing circuitry further comprises: a first channelthat is configured to provide output values responsive to a first subsetof the detection signals indicating the respective times of arrival ofthe plurality of photons comprising the signal photons and thebackground photons; a second channel that is configured to providereference values responsive to a second subset of the detection signalsindicating the respective times of arrival of the background photons;and a control circuit that is configured to calculate an estimate of theaverage time of arrival of the photons based on a relationship betweenthe output values and the reference values.
 4. The LIDAR apparatus ofclaim 1, wherein the processing circuitry is integrated in a same chipor package with the detector array, optionally wherein the single-photondetectors are single-photon avalanche detectors (SPADs).
 5. The LIDARapparatus of claim 4, further comprising: a control circuit that isconfigured to generate the respective strobing signals and/or calculatean estimate of the average time of arrival of the photons, optionallywherein the control circuit is integrated in the same chip or packagewith the detector array.
 6. A Light Detection And Ranging (LIDAR)measurement device, comprising: a detector array comprisingsingle-photon detectors that are configured to output respectivedetection signals indicating respective times of arrival of photonsincident thereon, wherein the photons comprise signal photons havingwavelengths corresponding to optical signals output from a pulsed lightsource; and processing circuitry comprising a recharge circuit that isconfigured to activate and deactivate subsets of the single-photondetectors for respective strobe windows between pulses of the opticalsignals and at respective delays that differ with respect to the pulses,responsive to respective strobing signals.
 7. The LIDAR measurementdevice of claim 6, wherein durations of the respective strobe windowsdiffer.
 8. The LIDAR measurement device of claim 7, wherein a timebetween the pulses of the optical signals corresponds to a distancerange, and wherein the durations of the respective strobe windows differaccording to sub-ranges of the distance range, optionally wherein thedurations of the respective strobe windows corresponding to closersub-ranges of the distance range relative to the LIDAR measurementdevice are greater than the durations of the respective strobe windowscorresponding to farther sub-ranges of the distance range relative tothe LIDAR measurement device.
 9. The LIDAR measurement device of claim6, wherein the recharge circuit is configured to activate and deactivatethe subsets of the single-photon detectors for the respective strobewindows responsive to the respective strobing signals based on relativepositions of the subsets of the single photon detectors in the detectorarray, optionally wherein the relative positions correspond to differentazimuths and altitudes.
 10. The LIDAR measurement device of claim 6,wherein the recharge circuit is configured to dynamically adjust thedurations of the respective strobe windows responsive to the respectivestrobing signals, optionally so as to alter boundaries of the sub-rangescorresponding to the respective strobe windows, or based on a brightnessof a target indicated by previous detection signals.
 11. A LightDetection And Ranging (LIDAR) measurement device, comprising: a detectorarray comprising single-photon detectors that are configured to outputrespective detection signals indicating respective times of arrival of aplurality of photons incident thereon, wherein the photons comprisesignal photons having wavelengths corresponding to optical signalsoutput from an emission source and background photons having wavelengthscorresponding to at least one other light source; and processingcircuitry configured to receive the respective detection signals outputfrom the single-photon detectors, wherein the processing circuitrycomprises: a time processing circuit comprising a counter circuitconfigured to increment a count value responsive to the respectivedetection signals, and a time integrator circuit configured to generatean integrated time value based on the respective times of arrivalindicated by the respective detection signals with respect to areference timing signal, wherein a ratio of the integrated time value tothe count value indicates an average time of arrival of the photons. 12.The LIDAR measurement device of claim 11, wherein the processingcircuitry further comprises: a recharge circuit that is configured toactivate and deactivate subsets of the single-photon detectors forrespective strobe windows between pulses of the optical signals and atrespective delays that differ with respect to the pulses, responsive torespective strobing signals.
 13. The LIDAR measurement device of claim11, wherein the processing circuitry further comprises: a correlatorcircuit that is configured to receive the respective detection signalsand output respective correlation signals representing detection of oneor more of the photons having a respective time of arrival within apredetermined correlation time relative to that of at least one other ofthe photons, wherein the counter circuit is configured to increment thecount value responsive to a subset of the respective detection signalscorresponding to the correlation signals, and the time integratorcircuit is configured to integrate the respective times of arrivalindicated by the subset of the respective detection signalscorresponding to the correlation signals.
 14. The LIDAR measurementdevice of claim 11, further comprising: a tunable optical filter elementarranged to transmit the photons that are incident on the detectorarray, the tunable optical filter element having a transmission bandthat is configured to vary based on a spectrum of the transmittedoptical signals and/or temperature of the emission source.
 15. The LIDARmeasurement device of claim 11, wherein the time processing circuitcomprises a first channel that is configured to provide the count valueand the integrated time value responsive to a first subset of thedetection signals indicating the respective times of arrival of theplurality of photons comprising the signal photons and the backgroundphotons, and a second channel that is configured to provide a referencecount value and a reference integrated time value responsive to a secondsubset of the detection signals indicating the respective times ofarrival of the background photons, and further comprising: a controlcircuit that is configured to calculate an estimate of the average timeof arrival of the photons based on relationships between the integratedtime value and the reference integrated time value, and between thecount value and a reference count value.
 16. A Light Detection AndRanging (LIDAR) measurement device, comprising: a detector arraycomprising single-photon detectors that are configured to outputrespective detection signals indicating respective times of arrival of aplurality of photons incident thereon; and processing circuitryconfigured to receive the respective detection signals output from thesingle-photon detectors, wherein the processing circuitry comprises: acorrelator circuit that is configured to output respective correlationsignals representing detection of one or more of the photons whoserespective time of arrival is within a predetermined correlation timerelative to that of at least one other of the photons.
 17. The LIDARmeasurement device of claim 16, wherein the correlator circuit isconfigured to output the correlation signals independent of stored dataindicating the respective times of arrival based on the detectionsignals, optionally without storing the respective times of arrival inone or more histograms.
 18. The LIDAR measurement device of claim 16,wherein the predetermined correlation time is relative to a leading edgeof the respective detection signal indicating the respective time ofarrival for the one or more of the photons, optionally wherein thepredetermined correlation time corresponds to a pulse width of opticalsignals output from a pulsed light source.
 19. The LIDAR measurementdevice of claim 16, wherein the correlator circuit comprises: respectivebuffer elements that are configured to delay the respective detectionsignals by the predetermined correlation time and output respectivepulsed signals having pulse widths corresponding to the predeterminedcorrelation time; and logic circuits that are configured output thecorrelation signals when the pulse widths of at least two of therespective pulsed signals overlap in time.
 20. The LIDAR measurementdevice of claim 16, wherein the processing circuitry further comprises:a time processing circuit comprising a counter circuit configured toincrement a count value responsive to each of the correlation signals,and a time integrator circuit configured to generate an integrated timevalue with respect to a reference timing signal based on the respectivetimes of arrival corresponding to the correlation signals, wherein aratio of the integrated time value to the count value indicates anestimated average time of arrival of the photons, optionally wherein theprocessing circuitry is configured to bypass the correlator circuit andprovide the respective detection signals to the time processing circuitbased on the respective detection signals relative to a predeterminedthreshold.
 21. The LIDAR measurement device of claim 20, wherein thetime processing circuit comprises a first channel that is configured toprovide the count value and the integrated time value responsive to thecorrelation signals, and a second channel that that is configured toprovide a reference count value and a reference integrated time valueresponsive to respective detection signals corresponding to photonswhose respective times of arrival are outside the predeterminedcorrelation time relative to one another, optionally wherein thecorrelator circuit is configured to increase or decrease thepredetermined correlation time when the respective detection signalscorresponding to photons whose respective times of arrival are outsidethe predetermined correlation time relative to one another are below athreshold.
 22. The LIDAR measurement device of claim 16, wherein theprocessing circuitry further comprises: a recharge circuit that isconfigured to activate and deactivate subsets of the single-photondetectors for respective strobe windows between pulses of opticalsignals output from a pulsed light source and at respective delays thatdiffer with respect to the pulses, responsive to respective strobingsignals.
 23. The LIDAR measurement device of claim 16, furthercomprising: a tunable optical filter element arranged to transmit thephotons that are incident on the detector array, the tunable opticalfilter element having a transmission band that is configured to varybased on a spectrum of optical signals output from a pulsed light sourceand/or a temperature of the pulsed light source.
 24. A Light DetectionAnd Ranging (LIDAR) measurement device, comprising: a tunable opticalfilter element having a transmission band that is configured to varybased on a spectrum of optical signals output from an emission source atemperature of the emission source, and/or a temperature of the tunableoptical filter element; and a detector array arranged to receive outputlight transmitted through the tunable optical filter element, thedetector array configured to output respective detection signalsindicating respective times of arrival of a plurality of photonsincident thereon.
 25. The LIDAR measurement device of claim 24, furthercomprising: at least one actuator that is configured to alter a tiltangle of the tunable optical filter element relative to a referenceangle, wherein the tilt angle is continuously variable over apredetermined angular range or is variable among a plurality of discretetilt angles, and wherein the transmission band is configured to varybased on the tilt angle.
 26. The LIDAR measurement device of claim 25,further comprising: an impedance measurement circuit configured tomeasure respective impedances at respective regions of the tunableoptical filter element; and a driving circuit that is coupled to theimpedance measurement circuit and is configured to control the at leastone actuator to alter the tilt angle based on the respective impedances.27. The LIDAR measurement device of claim 24, wherein a temperature ofthe tunable optical filter element is configured to vary with thetemperature of the emission source, optionally wherein the tunableoptical filter element is thermally coupled to the emission source,comprises a substantially similar substrate as the emission source, iscomprised of one of more materials with a spectral temperaturecoefficient substantially similar to the of the emission source and/oris included in a temperature-controlled housing.
 28. A Light DetectionAnd Ranging (LIDAR) measurement device, comprising: a detector arraycomprising single-photon detectors that are configured to outputrespective detection signals indicating respective times of arrival ofphotons incident thereon, wherein the photons comprise signal photonshaving wavelengths corresponding to light output of an emission sourceand background photons having wavelengths corresponding to at least oneother light source; and processing circuitry configured to receive therespective detection signals output from the single-photon detectors,wherein the processing circuitry comprises: a first channel that isconfigured to provide output values responsive to a first subset of thedetection signals indicating the respective times of arrival of theplurality of photons comprising the signal photons and the backgroundphotons; a second channel that is configured to provide reference valuesresponsive to a second subset of the detection signals indicating therespective times of arrival of the background photons without the signalphotons; and further comprising: a control circuit that is configured tocalculate an estimate of the average time of arrival of the photonsbased on a relationship between the output values and the referencevalues.
 29. The LIDAR measurement device of claim 28, wherein thecontrol circuit is configured to sequentially operate one or more of thesingle-photon detectors of the detector array to provide the first andsecond subsets of the detection signals, optionally wherein the controlcircuit is configured to sequentially operate the one or more of thesingle-photon detectors to provide the second subset in coordinationwith deactivation of the emission source.
 30. The LIDAR measurementdevice of claim 28, wherein the control circuit is configured to operateone or more single-photon detectors of the detector array to provide thesecond subset concurrently with the first subset, wherein the one ormore of the single photon detectors comprises an optical filter thereonhaving a transmission band that is configured to prevent passage of thesignal photons to the one or more of the single-photon detectors. 31.The LIDAR measurement device of claim 28, wherein the processingcircuitry further comprises: a correlator circuit that is configured toreceive the respective detection signals and output respectivecorrelation signals representing detection of one or more of the photonswhose respective time of arrival is within a predetermined correlationtime relative to at least one other of the photons as the first subset,optionally wherein the correlator circuit is configured to increase ordecrease the predetermined correlation time when the second subset ofthe detection signals indicate that light from the at least one otherlight source is below a threshold.